Copyright: Mentor Software Inc, CS-MAP, Release 10.02
Citation: Copyright © 1998 Mentor Software, Inc.  All Rights Reserved


Coordinate Conversion System

The overview of the coordinate conversion system given below provides a basic understanding as to how the system works.  Descriptions of the individual interactive elements of the coordinate conversion system are then given.  Users rarely need to use these interactive components; but having this ability available for the unusual situation is comforting.

In This Chapter

Overview

Coordinate System Referencing

Coordinate System Key Name Conventions

Dictionary Entry Protection

Interactive Dialog Support

Interactive Dialogs

Supported Projections

Geodetic Reference

Geodetic Data Catalog


Overview

Conversion of coordinates from one coordinate system to another is a three step process, where the second step can have several ‘sub-steps’ to it.  While it might seem strange at first, we’ll first describe basic steps One and Three first, deferring Step Two.  After we’ve seen what Step One and Step Three do, we’ll explore how Step Two links One and Three together to form a complete coordinate conversion system.  How users provide these elements the information necessary to perform their functions is then described, followed by a description of a little programming trick used to enable this basic process to efficiently and effectively deal with geographic coordinates (i.e. latitude and longitude).  Finally, we wrap this overview up with a description of the naming convention the programmers used to name the thousand or so coordinate system definitions provided with the coordinate conversion system.

In This Section  Basic Step One   Basic Step Three  Basic Step Two  Providing Information  The Geographic Coordinate Trick	See Also  Coordinate Conversion System  Coordinate System Referencing   Coordinate System Key Name Conventions  Dictionary Entry Protection  Interactive Dialog Support  Interactive Dialogs  Supported Projections  Geodetic Reference  Geodetic Data Catalog


Basic Step One

Given a cartesian coordinate, the coordinate conversion system must first convert this coordinate to geographic form.  This is one of two places in the entire process where a projection is used.  Specification of a projection, in and of its self, is useless.  The parameters to the projection must also be made known to the projection mathemagics.  Thus, in step one, the user must essentially provide the coordinate conversion system with the:

•	projection; and

•	the projection parameters; and

•	the ellipsoid/sphere to be used, and;

•	the units of the cartesian coordinates.

Given this information, the coordinate conversion system can then convert the cartesian X's and Y's (eastings and northings if you prefer) to geographic coordinates.  This is little more than implementing the 'inverse' set of formulas of the projection.  In the jargon of projections and such, inverse refers to those formulae which convert cartesian X's and Y's back to geographic form.

We establish the convention that these internal, intermediary, geographic coordinates are always in degrees, referenced to Greenwich, and use positive numbers for east longitude and north latitude, use negative values for west longitude and south latitude, and wherever coordinate pairs are used, the longitude will appear first.  Using this convention is quite convenient and consistent with most CAD and GIS packages.

Notice, we have not said anything yet about the datum to which these internal, intermediary, geographic coordinates are referenced.  If we had to say something, the only thing we could say, somewhat accurately, is that these internal, intermediary, geographic coordinates are referenced to whatever datum the original cartesian coordinates were referenced to.

See Also

Overview

Basic Step Three

Basic Step Two

Providing Information

The Geographic Coordinate Trick


Basic Step Three

The final step in a coordinate conversion is the conversion of the internal, intermediary, geographic coordinates to cartesian form.  This is the second of two places in the entire process where a projection is used.  Specification of a projection, in and of its self, is useless.  The parameters to the projection must also be made known to the projection mathemagics.  Thus, in step three, the user must essentially provide the coordinate conversion system with the:

•	projection; and

•	the projection parameters; and

•	the ellipsoid/sphere to be used, and;

•	the units of the cartesian coordinates.

Of course, in this case the specifications provided must be those of the desired target coordinate system.  Given this information, the coordinate conversion system can then convert the internal, intermediary, geographic coordinates to cartesian X's and Y's (eastings and northings if you prefer).  This is little more than implementing the 'forward' set of formulas of the projection.  In the jargon of projections and such, forward refers to those formulae which convert geographic coordinates (i.e. latitude and longitude) to cartesian X's and Y's.

This process assumes, of course, that the geographic coordinates provided to it adhere to the convention for internal, intermediary, geographic coordinates established above.  This eliminated the need to specify, for example, the units of the geographic coordinates that were to be converted.  It also eliminated the specification of the prime meridian, the quadrant specifications, etc.  This becomes very convenient, as we will see later on.

Notice, we have not said anything yet about the datum to which the resulting cartesian coordinates are referenced.  If we had to say something, the only thing we could say, somewhat accurately, is that the resulting cartesian coordinates are referenced to whatever datum the internal, intermediary, geographic coordinates were referenced to.

See Also

Overview

Basic Step One

Basic Step Two

Providing Information

The Geographic Coordinate Trick


Basic Step Two

If we simply feed the results of Basic Step One to Basic Step Three, we can accomplish the conversion of coordinates from one cartesian system to another cartesian system.  However, what we have described so far will only work in the case where the two coordinate systems share the same geodetic reference system, i.e. they share the same datum.  So, naturally, the purpose of Basic Step Two is to perform any datum shift that may be necessary.

We have conveniently arranged for Step One to produce a geographic coordinate.  We have also arranged, quite conveniently, for Step Three to accept a geographic coordinate.  Also, quite conveniently, we have arranged for these geographic coordinates to adhere to a very definitive convention, and thus require no specifications except two.  These specifications are simply the datum upon which the coordinates produced by Step One are based, and the datum upon which Step Three requires its coordinates to be based.  This information is simply the datum upon which the source cartesian coordinate system is based, and the datum upon which the target cartesian coordinate system is based.  Thus, we really have all the information necessary.  (If you're skeptical about this, hang on till we wrap this all up in about two more sections.)

There are several different mathematical methods used to convert geographic coordinates from one datum to another.  The author of this document has chosen to use the term technique; many others use the term transformation, some others use the term conversion.  In any case, these methods are very different from projections, they address a completely different problem.  However, as do projections, these techniques require some parametric information.  We'll see how this information is provided below.

In converting from one cartesian system to another, it was very convenient to use a common intermediary; an intermediary common to all projections.  That is, we used geographic coordinates; geographic coordinate which adhered to a very definitive convention.  In datum conversions, the same technique is used, but the common intermediary is geographic coordinates (which adhere to our specific convention) and are referenced to the World Geodetic System of 1984, i.e. WGS84, the geodetic reference system upon which GPS (Global Positioning System) is based.

Thus, the basic Step Two is often a two 'sub-step' process.  That is:

1.	convert the internal, intermediary, geographic coordinates from the datum of the source coordinate system to geographic coordinates (still very much internal and intermediary, but I'm sure your getting tired of reading that) based on WGS84, and

2.	then converting the WGS84 geographic coordinates to geographic coordinates based on the datum to which the target coordinate system is based.

Alternatively, if the source datum and the target datum are the same, basic Step Two can become a no 'sub-step' process as it doesn't really need to do anything.  In the jargon of computer programming, in this case, Step Two does nothing, silently and quickly.

Finally, it is also entirely possible that a technique for conversion directly to WGS84 is not possible.  That is, a conversion from datum XXX to WGS84 is not known, but a conversion from XXX to, lets say, NAD27 is known.  In this case, Step Two may have to perform several 'sub-steps" to: 1) convert from XXX to NAD27; 2) convert from NAD27 to NAD83; 3) convert from NAD83 to WGS84; and then finally, 4) convert from WGS84 to the target datum.  Of course, it is possible that several steps may be necessary to get from WGS84 to the target as well.  By now you have the idea.

Step Two can be simply nothing, or a very convoluted series of 'sub-steps'.  However, in all cases, the basic result is the same: Using our rigid convention for internal, intermediary, geographic coordinates, convert the coordinates generated by basic Step One to the coordinates required by basic Step Three.

See Also

Overview

Basic Step One

Basic Step Three

Providing Information

The Geographic Coordinate Trick


Providing Information

Each of the basic three steps required some information in addition to the specific coordinates that are to be converted.  Step One requires information about the source coordinate system.  We briefly listed some of the information required above.  Note that the actual content and volume of this information is dependent upon the projection used.  Many of the parameters required are rather arcane numbers that are difficult to remember.  Basic Step Three requires the same volume and detail of information concerning the target coordinate system.  The means by which this information is provided to the coordinate conversion system is essentially the user interface.

The designers of the coordinate conversion system have developed a system of dictionaries to provide a simple means of addressing this information.  Entries in the ellipsoid dictionary, for example, contain all the necessary information about each ellipsoid supported by the system, and assign to each entry a key name.  Thus, one needs only supply the key name to access the complete description of desired ellipsoid.  Similarly for datums; only the key name is required to access the complete description of a datum.

More importantly, the same concept applies to coordinate system definitions.  Entries in the Coordinate System Dictionary contain a complete description of each coordinate system, including the key names of the datums and/or ellipsoids as required.  You, the user, can supply all the information which basic Step One needs to know about the source coordinate system simply by providing the key name assigned to the appropriate definition.  In the same manner, you can provide all of the information that basic Step Three needs by providing the key name assigned to the target coordinate system.  Basic Step Two gets the information it requires by simply extracting the name of the referenced datum from the two referenced definitions.

You can, therefore, simply specify the most complex of coordinate conversions by simply providing the key names assigned to the two coordinate systems involved.  To convert a coordinate from NAD27 UTM Zone 13 in meters to NAD83 Colorado State Planes, Central Zone, in Meters, for example, the user need only provide two key names: "UTM27-13" and "CO83-C".  No need to know anything about the central meridians, the scale reductions, standard parallels, origin latitudes, false northings, the polar radius, etc., etc.  The coordinate conversion system uses the provided key names to obtain all of this information from the respective dictionaries, and pass the appropriate information on to each of the basic steps described above.

Since the distribution dictionaries include most all commonly used definitions, most users need only learn a few key names to use the system effectively.  For the rare case where the distribution dictionaries do not contain the definitions required, interactive editors are provided to enable new definitions to be added.  For the experienced user, the editors also provide the ability to examine the distribution definitions for accuracy and compatibility with other systems.

See Also

Overview

Basic Step One

Basic Step Three

Basic Step Two

The Geographic Coordinate Trick


The Geographic Coordinate Trick

We have described in detail how the coordinate conversion system converts coordinates from one cartesian system to another.  However, it is frequently necessary to convert from cartesian coordinates to geographic coordinates or vice versa.  It is also frequently necessary that we convert to/from geographic coordinates that do not adhere to the rather rigid convention established for our internal, intermediary, geographic coordinates.  Can we accomplish this using the design described so far.  Yes we can with one little trick.

The trick is to implement a projection that does nothing, or almost nothing.  That is, a projection which, during basic Step One, will recognize that the source coordinate is already geographic and do nothing quickly and silently.  (Well almost nothing as we shall soon see.).  Given such a projection, we can then make an entry in our dictionary that indicates this projection is to be used, and assign it a key name, say "LL", just like any other coordinate system definition.  Users can now instruct the coordinate conversion system that the source coordinates are already geographic by providing "LL" as the key name for the source coordinate system.  Of course, users could also specify "LL" as the target coordinate system, and cause results to be geographic.

Since the "LL" entry exists in dictionary, there is no reason why it can't be referenced to a datum in exactly the same manner as all other types of coordinate systems.  So, we have "LL27" which is referenced to NAD27, "LL83" which is referenced to NAD83, and several other similar definitions.  This means that basic Step Two can still perform as advertised without any change to the system.  Also, the definitions that are based on this "pseudo" projection which does nothing can have units assigned to them, and can have an origin longitude other than Greenwich.  Thus, for example, a definition that accepts or produces geographic coordinates based on grads and referenced to the Meridian of Paris are possible.  Use the Coordinate System Editor (described below) to examine the coordinate system definition with key name "LL-IGN" for an example.

See Also

Overview

Basic Step One

Basic Step Three

Basic Step Two

Providing Information


Coordinate System Referencing

As mentioned above, coordinate system definitions may reference a datum or an ellipsoid. In any case, all coordinate system definitions must contain a reference to one or the other.  Converting from the angular measurements of latitude and longitude (usually expressed in terms of degrees) to the linear measurements of a Cartesian system (usually in meters or feet) requires a basic parameter: the radius of the subject.  Thus even the simplest projection needs to know the radius of the body being mapped in order to convert properly.  Within the framework of coordinate conversion system there are two methods for specifying the radius.

First, the coordinate system may be referenced directly to an ellipsoid.  This is accomplished by entering the appropriate ellipsoid key name in the field provided in the coordinate system definition.

Since all datums must be referenced to an ellipsoid, you can also specify the ellipsoid by referencing a datum.  This is accomplished by entering the datum key name into the appropriate field in the coordinate system definition.

In the second case, we obviously impute more information to the coordinate system definition.  Not only are we providing the absolutely necessary radius information, but we are also specifying the datum upon which the resulting geographic coordinates will be based.  Thus, it is always preferable to reference a datum in a coordinate system definition if that is possible.  Recognizing that this information is not always available, and for certain applications not necessary, the coordinate conversion system enables coordinate system definitions to reference an ellipsoid directly.

This discussion is expanded in the following 4 topics:

•	Geodetic Referencing

•	Cartographic Referencing

•	UTM Zone References

•	The LL Coordinate System

•	Default References

In This Section  Geodetic Referencing  Cartographic Referencing  UTM Zone References  The LL Coordinate System  Default References	See Also  Coordinate Conversion System  Overview  Coordinate System Key Name Conventions  Dictionary Entry Protection  Interactive Dialog Support  Interactive Dialogs  Supported Projections  Geodetic Reference  Geodetic Data Catalog


Geodetic Referencing

We shall refer to a coordinate system definition that is referenced to a specific datum as being geodetically referenced.  The coordinate conversion system infers from this reference that all geographic coordinates produced by this coordinate system definition are based on the referenced datum.  It is only when both the source and target coordinate systems are geodetically referenced that the coordinate conversion system can automatically perform datum shifts.  In all other cases, datum shifting is automatically disabled.

Wherever sufficient information was available to do so, the coordinate system definitions provided in the distribution are referenced to a specific datum and, therefore, are geodetically referenced.  The coordinate conversion system will automatically perform the appropriate datum shift when converting between any two of these coordinate systems.

See Also

Coordinate System Referencing

Cartographic Referencing

UTM Zone References

The LL Coordinate System

Default References


Cartographic Referencing

We shall refer to a coordinate system definition that is referenced directly to an ellipsoid as being cartographically referenced.  In such cases, the coordinate conversion system does not know which datum the intermediary geographic coordinates are based on.  Therefore, whenever a cartographically referenced coordinate system definition is involved in a conversion, all datum shifting is disabled.

See Also

Coordinate System Referencing

Geodetic Referencing

UTM Zone References

The LL Coordinate System

Default References


UTM Zone References

Several of the coordinate system definitions provided in the distribution are cartographically referenced.  For example, outside of North America, there is no definitive information that Mentor Software is aware of as to what datums UTM coordinates are based on.  Therefore, all UTM definitions provided by Mentor Software for zones outside of North America are cartographically referenced.  This means that each of these zones is referenced to a specific ellipsoid.  Again, there is no definitive information as to which ellipsoid is to be used.  This probably varies from one political subdivision to another.  In the distribution, all UTM zone definitions which are not referenced to a particular datum are cartographically referenced to the International 1924 ellipsoid.  You may need to change this reference to another ellipsoid in order to get the results appropriate for your project.

See Also

Coordinate System Referencing

Geodetic Referencing

Cartographic Referencing

The LL Coordinate System

Default References


The LL Coordinate System

The coordinate conversion system includes a definition of a cartographically referenced geographic coordinate system named "LL" which is especially useful.  It enables end users and application programmers to conveniently obtain "generic" latitude and longitude coordinates.  LL is a geographic coordinate system; that is, it is associated with the pseudo projection used for geographic coordinate systems.  Thus it returns latitude and longitude coordinates.  The definition specifies units of degrees and an origin of Greenwich; thus the returned values are fully consistent with the requirements of the internal, intermediary, geographic coordinates which were discussed above.  Finally, being cartographically referenced, this definition disables all datum shifts; thus the geographic results are always referenced to the same datum as the source coordinate system, even if that datum is unknown.

See Also

Coordinate System Referencing

Geodetic Referencing

Cartographic Referencing

UTM Zone References

Default References


Default References

Some applications that use the coordinate conversion system will provide the ability to use defaults.  Defaults apply only to the datum references, ellipsoid references, and unit portion of coordinate system and datum definitions.  Defaults should only be used by those very experienced in the nature of coordinate systems and geodetic datums.  Generally, the average user simply leaves all defaults turned off, and the normal application does not provide the means to activate a default.

Defaults were invented to address a specific problem.  Specifically, a given UTM zone, for example, may need to be referenced to any number of datums, and have any number of units.  But it is still the same UTM zone.  To accommodate this situation in the framework described above, it is possible that a sophisticated user may have many definitions of the same UTM zone, each with (let's say) a different datum.

The default concept enables the sophisticated user to say that, for example, the datum to which a coordinate system is referenced is a specific datum, but is defaultable.  That is, if a default datum value is active, the defaultable datum value is replaced by the currently active default value whenever the definition is used.  Thus, a sophisticated user could create a definition for a specific UTM zone with a defaultable datum reference and by simply changing the default value, change the datum reference from one datum to another.

The same feature applies to ellipsoid references in coordinate system definitions and the unit values upon which a coordinate system is based.  Refer to the help file and/or documentation to learn if defaults are supported by your application and, if so, how to activate and set them.

See Also

Coordinate System Referencing

Geodetic Referencing

Cartographic Referencing

UTM Zone References

The LL Coordinate System


Coordinate System Key Name Conventions

The coordinate conversion system derives much of its simplicity from the concept of saving complete coordinate system definitions to disk and identifying them with a key name.  A rather simple convention for assigning names to the various coordinate systems was followed when the first edition of the system was released.   While this convention has largely withstood the test of time, some historical artifacts still remain in the system.  In any case, the information in the following sections will be helpful in learning and remembering the key names associated with the coordinate systems you frequently need to use.

This discussion consists of the following four topics:

•	Key Name Requirements

•	Universal Transverse Mercator Zones

•	State Plane Coordinate System Zones

•	Geographic Coordinate Systems

In This Section  Key Name Requirements  Universal Transverse Mercator Zones  State Plane Coordinate System Zones  Geographic Coordinate Systems	See Also  Coordinate Conversion System  Overview  Coordinate System Referencing  Dictionary Entry Protection  Interactive Dialog Support  Interactive Dialogs  Supported Projections  Geodetic Reference  Geodetic Data Catalog


Key Name Requirements

While the coordinate conversion system will preserve case in key name, the key names themselves are case insensitive.  That is, two key names that differ only in the case of one or more letters are considered to be the same for searching and location purposes.  Also, keep in mind that all key names must:

•	start with a letter, the underscore, or the colon character;

•	include only letters, numbers, the dollar sign, hyphen, underscore, space, slash, and colon characters;

•	must not include a double space sequence;

•	will have trailing spaces trimmed automatically;

•	must not exceed 23 characters in length

•	include the colon character if a user defined definition.

The length of key names was originally to limited to 8 characters (the good old days).  Therefore, some perversions of the general conventions described below have been retained to avoid renaming a previously existing coordinate system.

See Also

Coordinate System Key Name Conventions

Universal Transverse Mercator Zones

State Plane Coordinate System Zones

Geographic Coordinate Systems


Universal Transverse Mercator Zones

The names assigned to UTM zones all begin with the three letter acronym UTM.  If a datum is associated with the zone, such as NAD27, and indication of this follows the UTM designation; e.g. UTM27, UTM83, etc.  The zone number is then given using a hyphen as a separator character; e.g. UTM27-13, UTM83-14.  For many northern hemisphere zones, there is a corresponding southern hemisphere zone.  In the above examples, the datum references pretty much imply North American, so no indication of the hemisphere is necessary in the name.  Outside of North America, however, an N or an S is appended to the name to indicate the hemisphere of the UTM zone.

Also in North America, it is not uncommon to use UTM coordinates in feet, especially U.S. Survey feet. These definitions are distinguished from the normal UTM zones (unit = meters) by appending an F to the end of the name following the zone number.  The following table demonstrates the naming convention for UTM zones by example.

Coordinate System Key Name	Description
UTM-31N	UTM Zone 31, Northern Hemisphere, No Datum Reference
UTM-22S	UTM Zone 22, Southern Hemisphere, No Datum Reference
UTM27-13	NAD27 Based UTM Zone 13, in Meters
UTM83-14	NAD83 Based UTM Zone 14, in Meters
UTMHP-16	HPGN (HARN) Based UTM Zone 16, in Meters
UTM27-13F	NAD27 Based UTM Zone 13, in U. S. Survey Feet
UTM83-14IF	NAD83 Based UTM Zone 14, in International Feet
UTM84-23S	WGS84 Based UTM Zone 23, Southern Hemisphere, in meters
UTM56-17S	PSAD56 Based UTM Zone 17, Southern Hemisphere, in meters
See Also

Coordinate System Key Name Conventions

Key Name Requirements

State Plane Coordinate System Zones

Geographic Coordinate Systems


State Plane Coordinate System Zones

All State Plane Coordinate Systems (SPCS) begin with the two character abbreviation used by the US Postal Service. In those states where there is more than one zone, a zone indicator is appended after a separating hyphen. With some exceptions, zone indicators are taken from the list shown in the following table.

Zone Indicator	Read
S	South Zone
SC	South Central Zone
C	Central
NC	North Central
N	North Zone
E	East Zone
EC	East Central Zone
WC	West Central
W	West Zone
For example, this manual is being written in the Central Zone of Colorado, or CO-C.  My friends to the north of me live in east central Wyoming, which would be WY-EC.  The SPCS of states with a single zone, such as New Hampshire and Tennessee, are referred to with the two character state code such as NH and TN.

In California, roman numerals are used for zone indicators, e.g. CA-I, CS-II, CA-IV, etc.  In Alaska, the zones are numbered 1 through 10, e.g. AK-1, AK-5, AK-10.  The Hawaiian zones are numbered 1 through 5, e.g. HI-1, HI-5.

There are two special zone indicators which are fairly logical: NY-LI for the Long Island zone of New York, and MA-IS for the Island Zone of Massachusetts.

Since the original edition of the coordinate conversion system was released prior to the advent of NAD83, the above described names apply to the NAD27 version of all state plane coordinate systems.  The NAD83 version is distinguished by inserting 83 immediately after the two character state abbreviation. For example: CO83-C, AK83-5, RI83.

The original, official, definition of all NAD83 State Plane Coordinate Systems specified the unit as the meter.  However, in actual usage, the U.S. Survey Foot remained popular and some states decided to include the International Foot as an acceptable unit.  Therefore, the unadorned NAD83 based State Plane Coordinate System definitions are based on meters.  For each such coordinate system, a variation exists in which the unit is the U. S. Survey Foot and is distinguished by appending an F to the name:  CO83-CF,  CA83-IIIF,  WA83-NF.  For those states where the International Foot has been deemed an acceptable unit, yet another variation exists in which the unit is the International Foot and is distinguished by appending IF to the name:  AZ83-WIF.

Note that as of this writing, High Accuracy Reference Network (HARN) data exists (or will soon exist) for all 48 conterminous states.  Therefore, for most all supported NAD83 zone, there exists a HARN zone.  Such coordinate systems are indicated by the two character sequence "HP" following the state postal code;  COHP-C;  WAHP-N,  WYHP-EC;  AZHP-EIF for example.  The use of "HP" is another historical artifact.  What is now referred to as HARN (High Accuracy Reference Network) was previously referred to as High Precision GPS Network (i.e. HPGN).  Thus, the convention adopted at the time was to use the "HP" sequence.

See Also

Coordinate System Key Name Conventions

Key Name Requirements

Universal Transverse Mercator Zones

Geographic Coordinate Systems


Geographic Coordinate Systems

Geographic coordinate system definitions can also include specifications as to the allowable range of longitude. Normally, longitude is expected to remain with the range of greater than -180 and less than or equal to +180. Geographic coordinate system definitions with the names LL-360 and LL-270 define geographic coordinate systems where longitude is specified to range from 0 to 360 and -270 to +270 respectively.

LL is the coordinate system name assigned to the generic geographic coordinate system. As its name implies, there is no datum reference. Zero longitude refers to the Greenwich prime meridian, and the unit is degrees. Thus, this coordinate system will always return the most commonly used form of geographic coordinates. Since there is no datum referenced, datum shifting is disabled. Thus, the geographic coordinates returned are always referenced to the same datum as the source. This is especially useful when one does not know the datum of the source coordinate system.

See Also

Coordinate System Key Name Conventions

Key Name Requirements

Universal Transverse Mercator Zones

State Plane Coordinate System Zones


Dictionary Entry Protection

 Entries in the Coordinate System, Datum, and Ellipsoid Dictionary are of significant value.  Without protection, it would be easy to corrupt and/or delete important definitions.  Once a definition has been around a while and, therefore, likely to have been used more than once or twice, it becomes important to be relatively confident of being able to retrieve the definition for addition use, or possibly just reference.  A protection system has been added to the coordinate conversion system to implement the ideas discussed above.  Disabling this protection system is possible, but information as to how this can be accomplished is provided only upon request to appropriate individuals.

In This Section  Key Name Duplication  Distribtution Definition Protection  User Definition Protection	See Also  Coordinate Conversion System  Overview  Coordinate System Referencing  Coordinate System Key Name Conventions  Interactive Dialog Support  Interactive Dialogs   Supported Projections  Geodetic Reference  Geodetic Data Catalog


Key Name Duplication

Updates to the coordinate conversion system software will often include an update to the distribution dictionaries, i.e. the data files that contain the coordinate system definitions, datum definitions, and ellipsoids definitions.  It is to our mutual advantage that this update process does not alter or interfere with user defined entries.  The means by which this is accomplished is by requiring that user defined entries in all dictionaries contain a colon in their name.  Distribution definitions will never have a colon in their name.

It is recommended that you establish, and follow, a convention with regard to the key names assigned to dictionary entries you define.  For example, if the name of your company is Accurate Maps, Inc., you might adopt the convention that all coordinate system names would begin with "AMI:".  In any event, by requiring user definitions to include a colon character, and avoiding the use of the colon character in the distribution dictionaries, there would never be a key name collision in dictionary entry definition names.

See Also

Dictionary Entry Protection

Distribtution Definition Protection

User Definition Protection


Distribtution Definition Protection

All distribution definition entries are protected.  No doubt this is somewhat painful for the experienced user.  However, service call logs indicate that this feature has spared inexperienced users enormous amounts of pain and grief.  If you attempt to modify a definition that was part of the original coordinate system definition distribution, you will be informed that the definition is protected and that you will not be able to save your changes.

See Also

Dictionary Entry Protection

Key Name Duplication

User Definition Protection


User Definition Protection

Once a user definition has remained unchanged for 60 days, the coordinate conversion system will automatically apply protection to the definition.  The idea here is that a coordinate system which has remained unchanged for 60 days is a valid definition and has been used a few times.  Therefore, it is important to maintain a record of this definition.  If you attempt to modify a definition that was part of the original coordinate system definition distribution, you will be informed that the definition is protected and that you will not be able to save your changes.

See Also

Dictionary Entry Protection

 Key Name Duplication

Distribtution Definition Protection


Interactive Dialog Support

Certain aspects of the interactive dialog are of a supporting nature and apply to all, or most, of the individual dialogs.  These aspects are described in this section, in the following four topics:

•	Numeric Data Entry

•	Dictionary Entry Protection

•	New Key Name Dialog

•	Coordinate System Browser

 •	Datum Reference Selector

•	Ellipsoid Reference Selector

In This Section  Numeric Data Entry  New Key Name Dialog  Coordinate System Browser  Datum Reference Selector  Ellipsoid Reference Selector  Data Directory Dialog  Geodetic Data Catalog Entry	See Also  Coordinate Conversion System  Overview  Coordinate System Referencing  Coordinate System Key Name Conventions  Dictionary Entry Protection  Interactive Dialogs  Supported Projections  Geodetic Reference  Geodetic Data Catalog


Numeric Data Entry

Several of the interactive dialogs presented by the coordinate conversion system require that you provide numeric input.  For your convenience, you may enter numeric values in any one of several forms described below to enter numeric values.  Valid forms of numeric data entry are best shown by example.  Refer to the tables shown below.

Note, that when entering X/Easting and/or Y/Northing coordinates, thousands separators (i.e. the comma) may be used.  However, it used, the separator must be used consistently and accurately as a thousands separator or an error condition will be reported.  You may use either the plus/minus sign, or the directional characters, to indicate the sign of the value.  Using both is considered to be ambiguous and causes an error condition.

Valid Entry	Comments
500000.00	Standard format, decimal point is not required for this value.
500,000.00	Commas can be used, but must be properly positioned
500,000.00E	Directional character E indicates positive value; X coordinates only
500,000.00W	Directional character W indicates negative value; X coordinates only
1,000,000.00S	Directional character S indicates negative value; Y coordinates only
1,234,567.89N	Directional character N indicates positive value, Y coordinates only
-1.23456789E+06	Exponential notation is supported.
	Blank fields are considered zero.
Note that you may input either decimal degrees, degrees and minutes, or degrees, minutes, and seconds.  The space character is used to separate minutes from degrees, and seconds from degrees.  A numeric entry that contains a single space (the space being neither leading or trailing) causes the entered values to be parsed as a degree and minutes specification.  Similarly, if two spaces are entered, degrees, minutes, and seconds form is assumed.  You may use either the plus/minus sign, or the directional characters, to indicate the sign of the value.  Using both is considered to be ambiguous and causes an error condition.

Valid Entry	Comments
-123.456789	Decimal degrees
123.456789W	Decimal degrees, directional character W indicates negative value, Longitude only
123.456789E	Decimal degrees, directional character E indicates positive value, Longitude only
45  30  35.34N	Degrees, minutes, and seconds form, directional character N indicates positive value, Latitude only
45  56.12345S	Degrees and minutes form, directional character S indicates negative value, Latitude only
105  07  12	Degrees, minutes, and seconds form; no sign indication produces positive result
-105  07	Degrees and minutes form, negative value indicates west longitude
45  5  6.123456N	Degrees, minutes, and seconds form with high precision, N indicates positive latitude value
See Also

Interactive Dialog Support

New Key Name Dialog

Coordinate System Browser

Datum Reference Selector

Ellipsoid Reference Selector

Data Directory Dialog

Geodetic Data Catalog Entry


New Key Name Dialog

Creation of a new dictionary entry requires that the user provide a new, unique, key name to be assigned to the entry.  A single dialog that performs this function is used in several different places.  The dialog presented is similar to that show immediately below.  Whenever you are presented with this dialog, note that you are being asked to:

1.	provide a key name which must (usually) contain a colon character, and

2.	provide a key name which is not already assigned to an existing definition.

Use the Preserve existing definition data for editing check box to make the creation of new definitions easier.

To make a new variation of an existing coordinate system definition, for example, first display the existing definition (quite possibly a protected distribution definition) that is to be modified.  Then click the New button.  In the resulting New Key Name dialog, enter the new key name and check the Preserve existing definition data for editing box before dismissing the dialog.  The editor (the coordinate system editor in this example) will then essentially copy all of the data elements of the previously displayed dictionary entry (coordinate system definition in this example) to the new entry being created.  You need only change the specific item(s) that are different before saving the new definition.

See Also

Interactive Dialog Support

Numeric Data Entry

Coordinate System Browser

Datum Reference Selector

Ellipsoid Reference Selector

Data Directory Dialog

Geodetic Data Catalog Entry


Coordinate System Browser

This dialog is made available through a browse button (often a small button with a ‘B’ as the label) wherever a coordinate system name is required.  Use this dialog to lookup the key name of a coordinate system that you don’t happen to remember.  Since there are more than a thousand different coordinate system definitions in the dictionary, it is often faster and more convenient to just type the name in if you do know the appropriate key name.

The coordinate system browser breaks the task of locating a coordinate system down to two separate tasks.  First, you will need to select the group to which the coordinate system belongs (or at least the group you think it belongs to).  Use the Group drop down list to select a group from those listed.

Once the group has been selected, the Key Name drop down list will include the key names of all coordinate systems that belong to the selected group.  As you peruse this drop down list, pertinent information concerning the coordinate system is displayed in the data area of the dialog box.  Use this information to verify that the key name selected is indeed the key name of the desired coordinate system.

Click Select to cause the current key name to be copied to the coordinate system name field of the original dialog box.  Of course, you can click Cancel to terminate the browser.

See Also

Interactive Dialog Support

Numeric Data Entry

New Key Name Dialog

Datum Reference Selector

Ellipsoid Reference Selector

Data Directory Dialog

Geodetic Data Catalog Entry


Datum Reference Selector

The coordinate conversion system's coordinate system editor will display a dialog similar to that shown below when you request to specify a new or different datum reference.

Use the Key Name drop down list to choose the datum that you wish to have your coordinate system definition reference.  Use the data displayed on the dialog to verify that the key name selected does indeed reference the specific datum you wish to reference.  Choose Select to cause the currently displayed datum to be set as the selected datum in your coordinate system definition.  Choose Cancel to terminate the dialog without making a selection.

See Also

Interactive Dialog Support

Numeric Data Entry

 New Key Name Dialog

Coordinate System Browser

Ellipsoid Reference Selector

Data Directory Dialog

Geodetic Data Catalog Entry


Ellipsoid Reference Selector

The coordinate conversion system's coordinate system editor and datum editor will display a dialog similar to that shown below when you request to specify a new or different ellipsoid reference.

Use the Key Name drop down list to choose the ellipsoid that you wish to have your coordinate system or datum definition reference.  Use the data displayed on the dialog to verify that the key name selected does indeed reference the specific ellipsoid you wish to reference.  Choose Select to cause the currently displayed ellipsoid to be set as the selected ellipsoid in your coordinate system or datum definition.  Choose Cancel to terminate the dialog without making a selection.

See Also

Interactive Dialog Support

Numeric Data Entry

New Key Name Dialog

Coordinate System Browser

Datum Reference Selector

Data Directory Dialog

Geodetic Data Catalog Entry


Data Directory Dialog

The coordinate conversion system requires access to a large number of data files including, among others, the three files which carry the three dictionaries.  It is an essential part of the design that the coordinate conversion system be able to locate these data files without assistance from the operator each time coordinate conversion is required.  Most applications which use the coordinate conversion system provide for the installation of the appropriate data files in a location know to the application.  The coordinate conversion system, therefore, knows where to find them.

Your system may use more than one application which uses the same coordinate conversion system.  You may, therefore, consider it desirable for all such applications to access the same set of dictionary files.  Applications therefore, often supply access to this dialog so that you, the user, can specify the specific directory (and the exact files names for that matter) where the dictionaries and other data files are located.

Since this dialog is rarely used, there is no browse button.  You must key in the entire path to the supporting data directory.  You can also specify the names of the files (i.e. file names) that are to be used.  This can be useful for an administrator who needs to define, and test, new coordinate systems before making them generally available to the user community.

See Also

Interactive Dialog Support

Numeric Data Entry

New Key Name Dialog

Coordinate System Browser

Datum Reference Selector

Ellipsoid Reference Selector

Geodetic Data Catalog Entry


Geodetic Data Catalog Entry

While the coordinate conversion system's Geodetic Data Catalog Editor enables users to manipulate entries in the catalog, it is the Geodetic Data Catalog Entry Dialog which enables users to enter/modify the individual elements of such an entry.  When appropriate, the coordinate conversion system will display the Geodetic Data Catalog Entry Dialog, which looks something like this:

Use this dialog to modify the individual elements of an entry as described below.

•	File Name: The actual file name, and the file name's extension, of the entry is displayed in this field.  Use this field to enter the name of the file which is to be referenced by this entry.  While you can key in the file name, it is usually more convenient (and accurate) to use the Browse button provided.  Note that for the .las/.los file pairs (e.g. conus.las and conus.los), the extension needs to be set to .l?s.  The browser will do this automatically for you.  Use of the File Name Browse button will also cause the path to the file to entered into the File Path field which is next described.

•	File Path:  The drive/directory path to the file is displayed in this field.  While you can key in the entry manually, it usually best to simply use the File Name Browse button which will accurately fill in the value for this field.  Note that this field will always display the full path to the file.  You should not try to put a relative path in this field.

•	Force absolute reference: References to datum shift grid files can be stored in absolute or relative form.  The absolute form usually looks like that shown in the image above.  (Universal Naming Convention (UNC) form of an absolute reference is also supported.)  Relative references take the form of a '.' or a '..'.  That is, a 'dot', or a 'double dot'.  The 'dot' is interpreted to be shorthand for full path to the directory in which the Geodetic Data Catalog resides.  The 'double dot' is interpreted to be shorthand for the full path to the parent directory of the directory in which the Geodetic Data Catalog resides.  The coordinate conversion system will (normally) determine if relative referencing can be used and use it whenever appropriate.  Check this box to turn this feature off, thereby forcing the reference to the file to be stored in absolute form.  [Note: relative referencing makes it easy to move the catalog and the referenced files to different directories while absolute referencing is useful when a user desires to share a single copy of a referenced file among several different applications.]

•	Buffer Size: The coordinate conversion system automatically chooses a file data buffer size which is appropriate for a specific file.  This size tends to be a minimum size which is consistent with efficient use of system resources.  Users can override this automatic determination by entering a value in this field.  Users can then cause the coordinate conversion system to use a large buffer size (i.e. increase performance) for the specific files which they know will be used frequently.  Set the buffer size to zero to return to the automatic mode.

•	Grid Cell Size: The size of the grid cell within the datum shift grid file is one of the criteria the coordinate conversion system uses to select a specific data file for use.  While the grid cell size can be, and is, computed from data in the file, users can use this field to override the result of the automatic calculation.  Thus, users can obtain fine manual control over the normally automatic process of datum shift grid file selection.  Note, that the size of a grid cell is specified in minutes of longitude/latitude.  Enter a zero to have the coordinate conversion system use the calculated value.

•	Options: Geodetic Data Catalog format specifications support the specification of several options.  As of this writing, no such options have been implemented.  In future releases, check boxes or radio buttons in this area of the dialog will enable users to selectively turn these options on and off.

•	Commit: Click the commit button to dismiss the dialog and cause the changes made to be recorded in the currently active Geodetic Data Catalog file.

•	Cancel: Click the Cancel button to dismiss the dialog without changing the contents of the currently selected Geodetic Data Catalog.

See Also

Interactive Dialog Support

Numeric Data Entry

New Key Name Dialog

Coordinate System Browser

Datum Reference Selector

Ellipsoid Reference Selector

Data Directory Dialog


Interactive Dialogs

The coordinate conversion system provides five interactive dialogs which can be used to maintain the system.  Under normal circumstances, users rarely need to use any of these dialogs.  However, it is comforting to know that this ability is available.

The discussion of the interactive dialogs is accomplished in the following five topics:

•	Data Directory Dialog

•	Coordinate System Editor

•	Datum Editor

•	Ellipsoid Editor

•	Coordinate System Test Dialog

In This Section  Coordinate System Definition Editor  Datum Editor  Ellipsoid Editor  Geodetic Data Catalog Editor  Coordinate System Test Dialog  MGRS Test Dialog	See Also  Coordinate Conversion System  Overview  Coordinate System Referencing  Coordinate System Key Name Conventions  Dictionary Entry Protection  Interactive Dialog Support  Supported Projections  Geodetic Reference  Geodetic Data Catalog


Coordinate System Definition Editor

Use the Coordinate System Editor to examine, modify, create, and delete coordinate systems.  Upon activation, you are presented with a standard Windows tabbed dialog box, also known as a property sheet.  Each page of the tabbed dialog box is described in the following sections.  Of immediate importance are the four buttons on the bottom of the dialog box.  These control the operation of the editor and the labels and functions of these buttons change as the state of the editor changes.

•	Use the Close button to close the dialog box when finished with it.

•	Use the New button to create a new definition.

•	Use the Save button to save changes made to an existing or new definition.  New definitions are not actually created until 'saved' in this manner.

•	Use the Cancel button to cancel all changes made to a definition, or cancel the addition of a new definition.

•	Use the Delete button to delete the currently displayed coordinate system.

•	Use the Help button to obtain help on using the editor.

Immediately upon sensing a change to the currently displayed coordinate system, the New and Delete buttons are replaced with Save and Cancel buttons; thus indicating that your changes to the existing coordinate system definition must be either Save'ed or Cancel'ed before you will be permitted to define a new coordinate system, or delete an existing coordinate system.

In This Section  Identification Page  General Page  Origins Page  Extents Page  Parameter Page(s)	See Also  Interactive Dialogs  Datum Editor  Ellipsoid Editor  Geodetic Data Catalog Editor  Coordinate System Test Dialog  MGRS Test Dialog


Identification Page

Use the Identification page of the tabbed Coordinate System Editor dialog box to select the specific coordinate system definition you wish to examine, modify, or delete.  This page also shows the information usually used to identify a specific coordinate system and hence the name assigned to this tab.  This page also provides the facilities for entering/modifying this information.

The edit fields are used as follows:

•	Key Name is the actual key name assigned to the coordinate system definition.  This is the name by which you access the definition when performing coordinate conversions.  Obviously, the key name assigned to a given coordinate system definition must be different from the key names assigned to all other coordinate systems.  You may select a coordinate system for display/edit by selecting its name from the drop down list, or by clicking on the Browse button located to the right of this field.  Note, when perusing a drop down list, keying in a letter on the keyboard is a fast way to seek to entries in the list that start with that letter.

•	Description consists of descriptive text indicating the official title or the use and purpose of the coordinate system.  This information is not used by the coordinate conversion system in any way other than to display it at appropriate times to assist you in selecting coordinate systems and or assuring that a coordinate system definition selected by key name is indeed the coordinate system you desired.

•	Source refers to the source of the information used to define the coordinate system.  Again, the coordinate conversion system does not use this information in any way other than to display it at appropriate times.

•	Country/State is not used currently. In future releases this field may be used to encode the various countries and states in which use of this coordinate system is appropriate.  Countries will be encoded using the two character ISO code assigned to each country in upper case letters.  Each country code may be optionally followed by local state/province codes in lower case letters.  Spaces and other punctuation characters are ignored.

•	Locations is not used currently. In future releases this file will be used to encode the geographic areas, in a non-political manner, where use of this coordinate system is appropriate.

See Also

Coordinate System Definition Editor

General Page

Origins Page

Extents Page

Parameter Page(s)


General Page

The General page is used to display/modify the general characteristics of the coordinate system.  These characteristics include:

•	Projection:  Select the projection upon which the coordinate is based.  The coordinate conversion system currently supports thirty two projections and variations of several of these.

•	Referenced To:  Coordinate system definitions must be referenced to either a datum or an ellipsoid. The coordinate system editor uses this group box to indicate the type of reference, and the specific entity, to which the current coordinate system is referenced.  To change the reference, click on the appropriate box on the right hand side.  Selecting a new datum using the Select different Datum button will automatically change the type of reference to 'Datum' if necessary.  Similarly, the editor will, if necessary, change the type of reference to 'Ellipsoid' if a different ellipsoid is chosen as the coordinate system reference using the Select different Ellipsoid button.  References can be defaultable.  Defaults are useful only to experienced users, and only then in rare situations.  Refer to the section on Defaults for more information on this feature.  Do no check this box unless you are very comfortable with the information discussed there.

•	Unit:  Select the unit upon which coordinate system coordinates are to be based.  The available choices are provided in drop down list form.  Note that the unit named Foot refers to the U. S. Survey foot, the most commonly used unit of feet.  The unit named IFoot refers to the International Foot, a less frequently used, and slightly different, unit.  The choices available will switch to angular units when the chosen projection in the pseudo projection used for geographic coordinate systems.  Units can be defaultable.  Defaults are useful only to experienced users, and only then in rare situations.  Refer to the section on Defaults for more information on this feature.  Do no check this box unless you are very comfortable with the information discussed there.

•	Scale Reduction:  Scale Reduction is also referred to as the Scale of the Central Meridian.  Scale reduction is not applicable to all projections and this field is disabled (grayed out) for those projections for which it is not applicable.  (For the pseudo UTM projection, this field is grayed out to indicate that the user cannot change the preset scale reduction of 0.9996.)  For those projections where it is applicable use this field to enter the scale reduction factor for the coordinate system.  The actual numeric value is usually (but not necessarily) slightly less than or equal to one.  For example, the scale reduction use for all UTM zones is 0.9996.  The scale reduction, in the form of a ratio, is shown just to the right of this edit box.  You can enter the scale reduction factor in ratio form if you like.  To do so, be sure that the first two characters of your entry are '1:'.  These must then be followed by the appropriate integer value (which may include a thousands separator).  The editor will display the numeric form in the edit box, and the ratio form to the right upon completion of your entry.

•	Map (paper) Scale:  This value is almost always set to one (i.e. 1.0).  Other values are used only when working with coordinates on a scaled map.  For example, when designing a coordinate system that is to represent inches on a map scaled at 1:24,000, you would enter 24,000 in this field to indicate the scale of the map.  In this case, you would also need to set the Unit specification to Inch.  Again, ratio form may be used to enter the map scale, and is displayed to the right of the numeric value.

•	Quadrant:  In rare cases, coordinate systems need to be defined, for example, where X coordinates increase to the west, or Y coordinates increases to the south.  Occasionally, there is a need to be able to swap the axes.  The Quadrant field can be used to accommodate these rather rare situations.  This field consists of a drop down list which provides the following choices:

•	X/Lng increases to the east, Y/Lat increases to the north (normal)

•	X/Lng increases to the west, Y/Lat increases to the north

•	X/Lng increases to the west, Y/Lat increases to the south

•	X/Lng increases to the east, Y/Lat increases to the south

•	Y/Lat increases to the east, X/Lng increases to the north

•	Y/Lat increases to the west, X/Lng increases to the north

•	Y/Lat increases to the west, X/Lng increases to the south

•	Y/Lat increases to the east, X/Lng increases to the south

The standard distribution includes several coordinate system definitions which use this feature to provide a latitude and longitude system where west longitude is positive.  Use the coordinate system editor to look at the coordinate system with key name WPLL27 for an example of this use.

See Also

Coordinate System Definition Editor

Identification Page

Origins Page

Extents Page

Parameter Page(s)


Origins Page

Use the Origins page of the Coordinate System Editor to examine/modify the origins assigned to the coordinate system. Typically, two origins are provided:  Geographic Origin and the False Origin.

Coordinate System Origin

The Coordinate System Origin is given by the geographic coordinates of the point at which the projection mathematics produce a zero value for X and Y Cartesian coordinates before the application of the False Origin.  (There are some exceptions to this, most notably the oblique Mercator projections.)  This is specified in terms of the Origin Longitude and the Origin Latitude.  Both specifications must be given in degrees. The longitude value must be specified relative to the Greenwich Prime Meridian. Use negative values for west longitude and south latitude.

For several projections, the Origin Longitude is the same as the Central Meridian that is usually specified on the Parameter Page.  In these cases, the Origin Longitude field will be disabled (grayed out).  Enter the Origin Longitude as the Central Meridian on the Parameter page in these cases.  For several projections, the Origin Latitude field will also be disabled.  This is true when the Origin Latitude is required (by the projection) to be the equator or, in certain cases, is determined by parameters given on the Parameter page.

The False Origin

The False Origin consists of X and Y offsets which are added to all coordinates.  False Origins are often zero.  They are used for two reasons.  First, it is considered desirable to have all Cartesian coordinates to be positive numbers.  False Origins are often chosen, therefore, to be large enough to cause all X and Y coordinates in the useful range of the coordinate system to be positive.  Second, False Origins are often used in multiple-zone systems to identify the specific zone being used.  For example, in the 1983 version of Texas State Plane Coordinates, there are five zones, and the northing for each is used to make if very difficult to mistakenly use coordinates in the wrong zone.

Minimum Non-Zero Coordinate Values

The Minimum Non-Zero Coordinate values are used to preclude very small numbers in the output results. For example, in rare cases, it is possible for coordinate conversions to generate coordinate values very close to zero but not exactly zero; 1.2343510-27 for example.  When printed in a report, such numbers can be very distracting and in some cases cause severe problems.  The values provided in the Minimum Non-Zero Coordinate value fields indicate the threshold under which the coordinate conversion system will force small values for X and Y coordinates (longitude and latitude for a geographic coordinate system) to hard zeros.

See Also

Coordinate System Definition Editor

Identification Page

General Page

Extents Page

Parameter Page(s)


Extents Page

Use the Extents page to examine/modify the useful range limits of the coordinate system definition.  If values are not provided (i.e. simply left empty or set at zero for both minimum and maximum values) the coordinate conversion system will automatically compute rather liberal values for the useful range each time the coordinate system is used.  You can use this page to specify values to override the automatic values calculated by the coordinate conversion system.  Please note, specifications for either type of extent must be complete.  You cannot simply change a maximum specification and expect the coordinate conversion system to supply the minimum automatically.  If you supply any information at all, you must supply a complete specification.

To maintain performance at the highest possible levels, the coordinate conversion system checks coordinate values against the extents specified here only when, and to the degree, necessary or when requested for a specific operation.  That is, the default mode of operation for coordinate conversions is to convert as fast as possible and only check for coordinate which may cause a real number exception such as dividing by zero.  Various operations which involve coordinate conversion may have different default modes of operation.  Check the documentation of the feature you plan to use for more information.  If no information is provided, you can assume that minimal checking is the default.

See Also

Coordinate System Definition Editor

Identification Page

General Page

Origins Page

Parameter Page(s)


Parameter Page(s)

Usually at least one Parameter page is visible, but up to four parameter pages may be required to display all of the parameters associated with a specific projection.  Of course, the parameters shown at any given time are those required/supported by the projection of the currently selected coordinate system.  Each parameter is given its own label that indicates the purpose of the parameter.  These labels will change as the projection selected on the General page is changed to indicate the specific nature of the parameters appropriate for the selected projection.

Wherever a latitude is required as a parameter value, the latitude must be specified in degrees north of the equator.  South latitudes are must be indicated by entering a negative number or qualifying the input with the S directional character.

Similarly, wherever a longitude is required as a parameter value, the value must be entered in degrees, east of Greenwich.  That is, all longitudes entered on the parameter screen must be relative to Greenwich where positive values are used for longitude east of Greenwich, and negative values are used for values west of Greenwich.  The W directional character may be used to indicate west longitude as well.

Wherever an angle is required as a parameter value, the value must be entered in degrees east of north.  A negative value would indicate an azimuth west of north.  Again, the E and W directional characters may be used to indicate the proper sign of the value.

See Also

Coordinate System Definition Editor

Identification Page

General Page

Origins Page

Extents Page


Datum Editor

Use the Datum Editor to examine, modify, create, and delete entries in the Datum Dictionary.  Upon activation, you are presented with a standard Windows dialog box.  Of immediate importance are the four buttons on the bottom of the dialog box.  These control the operation of the editor and the labels and functions of these buttons change as the state of the editor changes.

•	Use the Close button to close the dialog box when finished using it.

•	Use the New button to create a new definition.

•	Use the Save button to save changes made to an existing or new definition.  New definitions are not actually created until saved.

•	Use the Cancel button to cancel all changes made to a definition, or cancel the addition of a new definition.

•	Use the Delete button to delete the currently displayed datum definition from the dictionary.

•	Use the Help button to obtain help on using the editor.

Information concerning the elements of a datum definition are displayed in the edit fields described below.  Of course, you use these same fields to modify or create a new definition.

•	Key Name:  Key name is the unique name assigned to a datum definition.  This name is what actually appears in the definition of a coordinate system that references this datum.  This name must uniquely identify the datum in the dictionary; that is, two datum definitions cannot have the same key name.

•	Group:  Currently unused, the group field may be used in future releases to group datum definitions into smaller groups for easier selection and use.

•	Description:  Description carries the official name of the datum, or a description of its use and purpose.  Descriptions often help resolve ambiguities about the many different datum definitions available.  The coordinate conversion system does not use the description field in any way other than displaying it appropriate times.

•	Source:  Use the source field to record the source of information used to define this datum.  The coordinate conversion system does not use the description field in any way other than displaying it appropriate times.

•	Ellipsoid:  All datums must be referenced to a specific ellipsoid.  To select a different ellipsoid than that which is currently shown, or to obtain a more definitive description of the ellipsoid shown, click the Change Ellipsoid button to the right of this field.  This will activate the Ellipsoid Reference Selector dialog.  If the ellipsoid you need is not listed within the Ellipsoid Reference Selector dialog, than it does not exist in the Ellipsoid Dictionary and the Ellipsoid Editor (next described) will need to be used to enter it.

•	Convert To WGS84 Via:  Use this field to choose which of the 16 techniques currently available is to be used to convert coordinates based on this datum to WGS84 coordinates.  The available selections are:

Bursa/Wolf Transformation:  To use the Bursa/Wolfe Transformation, all seven parameters in the lower half of the dialog box must be provided.  Note, that the Seven Parameter Transformation accepts the exact same types of parameters, and is very similar to the Bursa/Wolf transformation.  If you are not sure, you may need to convert some test coordinates using both of these techniques to assure that your definition references the correct technique.

HARN->NAD83/WGS84 via NADCON:  HARN is an acronym for High Accuracy Reference Network.  HPGN is an acronym for High Precision GPS Network.  Both of these terms refer to the same thing.  This 'thing' has also been referred to as NAD83/91.  This 'thing' is a rework of NAD83 with the aid of GPS technology, as GPS was not functional in 1983.  Note that this selection is useful only for real estate residing in the 48 conterminous United States. This technique requires the existence of a Geodetic Data Catalog file named Nad83ToHarn.gdc.  Refer to Geodetic Data Catalogs for more information.

This technique selection implies the use of the algorithms and data files of the U. S. National Geodetic Survey's NADCON program to effect the shift between NAD83 and HARN.  Since NAD83 and WGS84 are generally considered to be the same, no further operations are performed.

Is NAD83, No Shift Required:  Choosing this selection is, currently, the same as choosing WGS84.  That is, the coordinate conversion system considers NAD83 and WGS84 to be essentially the same thing.  This is a generally accepted position, and no conversion between NAD83 and WGS84 has been published by any governmental agency.  This option implies that the data is already referenced to WGS84 and no further datum shifts are necessary.  This entry is provided to enable the implementation of an NAD83 to WGS84 conversion technique should such ever become available and/or desirable.

Is WGS84, No Shift Required:  The selection made in this box determines the technique which is to be used to convert from the datum being defined to WGS84.  Choosing this technique instructs the coordinate conversion system to do nothing.  Thus, this selection is used to define the WGS84 datum, or any one of its very close neighbors.  In other words, this selection implies that the datum being defined is already referenced to WGS84 and datum shifting is not required.

Molodensky:  Choose this selection to use the Molodensky Transformation. To use the Molodensky, the Delta X, Delta Y, and Delta Z parameters will need to be known and provided in the edit fields provided.  Molodensky is the simplest, fastest, but least accurate of the nine techniques available.  However, the information necessary to perform a Molodensky transformation is the most generally available and, therefore, the Molodensky is quite frequently used despite its drawbacks.

Multiple Regression ala DMA:  This technique requires the existence of an .MRT file (Multiple Regression Transformation).  Such files are produced as a part of the coordinate conversion system development from information obtained from U. S. Defense Mapping Agency Technical Report 8350.2-B (December 1987).  Currently, users cannot examine, modify, or create new .MRT files.  This may change in the future.

This technique is usually, on average, accurate to within 2 meters.  Note that the .MRT file has the base name as the key name of the datum.  Therefore, for historical reasons, all datum definitions that use this technique have fixed key names which are 8 or less characters in length.

NAD27->NAD83 via NADCON:  This technique is appropriate for data based on the North American Datum of 1927 only.  The algorithm and data files used by the U. S. National Geodetic Survey's NADCON program and/or Geomatics Canada's National Transformation programs are used to convert data to the North American Datum of 1983.  Conversions, in these cases, produce numeric results identical to the programs published by the respective governmental agencies.  Be sure that your application has access to the appropriate data files.  This technique requires the existence of a Geodetic Data Catalog file named Nad27ToNad83.gdc.  Refer to Geodetic Data Catalogs for more information.

With regard to the Canadian National Transformation, users must obtain a copy of the data from Geomatics Canada or their provincial government.  The coordinate conversion system supports both version 1 and version 2.  If a version 2 data file is present, it is used; otherwise version 1 is used if an appropriate data file is present.  If neither data file is present, the US NADCON files will be used for regions where the Canadian and US systems overlap, and a Multiple Regression formula used elsewhere.  Note that the equivalent data files for the US are in the public domain and available on the Internet.

Seven Parameter Transformation:  To use the Seven Parameter Transformation, all seven parameters in the lower half of the dialog box must be provided.  Note, that the Bursa/Wolf Transformation accepts the exact same types of parameters, and is very similar to the Seven Parameter transformation.  If you are not sure, you may need to convert some test coordinates using both of these techniques to assure that your definition references the correct technique.

WGS72->WGS84 via DMA formula:  This selection refers to the World Geodetic System of 1972, and implies conversion by formulas published by the U. S. Defense Mapping Agency in Technical Report 8350.2-B, December 1987.

Three Parameter Transformation: Choose this selection to use the widely used Three Parameter Transformation. To use the Three Parameter, the Delta X, Delta Y, and Delta Z parameters will need to be known and provided in the edit fields provided.  The Three Parameter Transformation is very similar to the Molodensky, and the results are quite similar.  However, a different mathematical technique is used to achieve the results.

Six Parameter Transformation: Choose this selection to use the rarely used Six Parameter Transformation.  The Six Parameter Transformation is essentially the same as the Seven Parameter Transformation, except that the scale factor parameter is forced to zero, implying no scaling is to take place.  Naturally, all parameters but the Scale parameter must be specified.

Four Parameter Transformation: Choose this selection to use the almost never used Four Parameter Transformation.  The Four Parameter Transformation simply adds a scale factor to the widely used Three Parameter Transformation.  Naturally, the Delta X, Delta Y, and Delta Z parameters must be specified in addition to the Scale parameter.

AGD66 <--> GDA94 via Grid File: This technique is appropriate for data based on the Australian Geodetic Datum of 1966 (AGD66) only.  The algorithm and data files published by ICSM of Australia.  Currently there are separate data files for several Australian states that primarily use AGD66. Be sure that your application has access to the appropriate data files. This technique requires the existence of a Geodetic Data Catalog file named Agd66ToGda94.gdc.  Refer to Geodetic Data Catalogs for more information.

AGD84 <--> GDA94 via Grid File: This technique is appropriate for data based on the Australian Geodetic Datum of 1984 (AGD84) only.  The algorithm and data files published by ICSM of Australia.  Currently there are separate data files for several Australian states that primarily use AGD84. Be sure that your application has access to the appropriate data files. This technique requires the existence of a Geodetic Data Catalog file named Agd84ToGda94.gdc.  Refer to Geodetic Data Catalogs for more information.

NZGD49 <--> NZGD2K via Grid  File:  This technique is appropriate for data based on the New Zealand Geodetic Datum of 1949 (NZGD49) only.  The algorithm and data files published by Land Information New Zealand.  Currently there is a single data file for the entire extent of NZGD49.  Be sure that your application has access to the appropriate data files. This technique requires the existence of a Geodetic Data Catalog file named Nzdg49ToNzgd2K.gdc.  Refer to Geodetic Data Catalogs for more information.

ATS77 <--> NAD83 via Grid File: This technique is appropriate for data based on the Average Terrestrial System 1977 (ATS77) only.  The algorithm and data files published by Services New Brunswick.  Currently there is a single data file for the entire extent of ATS77.  Be sure that your application has access to the appropriate data files. This technique requires the existence of a Geodetic Data Catalog file named Ats77ToNad83.gdc.  Refer to Geodetic Data Catalogs for more information.

•	Molodensky Parameters:  When the Convert to WGS84 Via selection is either Molodensky, Bursa/Wolf, or Seven Parameter, you must enter appropriate values in the fields labeled Delta X, Delta Y, and Delta Z.  These values represent the shift of the center of the referenced ellipsoid to the WGS84 ellipsoid in meters.  To be more precise, WGS84 three dimensional Cartesian X, Y, and Z coordinates are obtained by adding the shifts provided in this group to the three dimensional Cartesian X, Y, and Z coordinates of the datum being defined.  All values must be in meters; this is the generally accepted convention for such numbers.

•	Bursa/Wolfe Parameters:  When the Convert to WGS84 Via selection is set to Bursa/Wolfe or Seven Parameter, you must enter the appropriate values in the fields labeled Rotate X, Rotate Y, Rotate Z, and Scale.  The rotate values indicate the amount of rotation, in seconds of arc, necessary about each of the respective axes to bring the datum being defined into alignment with WGS84.  Similarly, the scale parameter indicates, in parts per million, the scaling necessary to bring the datum being defined into alignment with WGS84.  That is, a minus 1 (-1.0) indicates an actual scaling factor of 0.999999.

See Also

Interactive Dialogs

Coordinate System Definition Editor

Ellipsoid Editor

Geodetic Data Catalog Editor

Coordinate System Test Dialog

MGRS Test Dialog


Ellipsoid Editor

Use the Ellipsoid Editor to examine, modify, create, and delete entries in the Ellipsoid Dictionary.  Upon activation, you are presented with a standard Windows dialog box.  Of immediate importance are the four buttons on the bottom of the dialog box.  These buttons control the operation of the editor and the labels and functions of these buttons change as the state of the editor changes.

•	Use the Close button to close the dialog box when finished using it.

•	Use the New button to create a new definition.

•	Use the Save button to save changes made to an existing or new definition. New definitions are not actually created until saved.

•	Use the Cancel button to cancel all changes made to a definition, or cancel the addition of a new definition.

•	Use the Delete button to delete the currently displayed ellipsoid definition

•	Use the Help button to obtain help on using the editor.

Information concerning the elements of a ellipsoid definition are displayed in the edit fields described below.  Of course, you use these same fields to modify or create a new definition.

•	Key Name:  Key name is the unique name assigned to an ellipsoid definition.  This name is what actually appears in the definition of a coordinate system definition, or a datum definition, which references this ellipsoid.  This name must uniquely identify the ellipsoid in the dictionary; that is, two ellipsoid definitions cannot have the same key name.

•	Group:  Currently unused, the group field may be used in future releases of the coordinate conversion system to group ellipsoid definitions into smaller groups for easier selection and use.

•	Description:  Description carries the official name of the ellipsoid, or a description of its use and purpose.  Descriptions often help resolve ambiguities about the many different ellipsoid definitions available.  The coordinate conversion system does not use the description field in any way other than displaying it at appropriate times.

•	Source:  Use the source field to record the source of information used to define this ellipsoid.  The coordinate conversion system does not use the description field in any way other than displaying it at appropriate times.

•	Radii:  Use these two fields to enter the equatorial and polar radii of the ellipsoid.  These values are also known as the semi-major axis and the semi-minor axis respectively.  The values must be given in meters.  You can define a sphere by causing the polar radius to be precisely the same as the equatorial radius.

•	Calculated Values:  Upon entry of reasonable values for the radii, the ellipsoid editor will calculate the square of the eccentricity and the reciprocal of the flattening and display the calculated results here.  Use these values to verify the correct entry/calculation of the radii.  Data cannot be entered into these fields.

See Also

Interactive Dialogs

Coordinate System Definition Editor

Datum Editor

Geodetic Data Catalog Editor

Coordinate System Test Dialog

MGRS Test Dialog


Geodetic Data Catalog Editor

Geodetic Data Catalogs are ordinary text files.  You can use any ordinary text editor, such as Notepad, to edit these files.  The contents of the catalog files are described elsewhere in this help file.  You may find it more convenient, however, to use the Geodetic Data Catalog Editor.  You may also use the Geodetic Data Catalog Editor to simply examine the contents of a catalog and determine that it is indeed set up in the manner desired.

Please note that the Geodetic Data Catalog was originally designed to support the use of multiple files for a single datum conversion.  Supporting both U. S. and Canadian data files for the NAD27 to NAD83 calculation, for example.  (NAD refers to North American Datum.)  The basic concepts of Geodetic Data Catalog files also apply to such things as vertical datum shifts and geoid height calculations.  Thus, while the documentation and labeling are oriented towards the horizontal datum shift problem, the same dialog can (and is) used to edit catalogs designed for use with vertical datums and geoid height data sets as well.

Once started, the Geodetic Data Catalog Editor will present a dialog which looks something like this:

Use the list box labeled Catalog Selector at the top of the screen to select the specific catalog you wish to edit or examine.  Once chosen, the central list box of the dialog labeled Entry File Names will show a list of the files that are referenced by this catalog.  Note that the Entry File Names list box only shows the file name and extension.  The full path to the file, and the options associated with that file are shown below in the group box labeled Selected Entry Details.  The editor also shows the Fallback Datum assignment if there is one.

Use the system pointing device to select a specific entry and the details will be updated.  Alternatively, use the up and down arrow keys to select an entry in the list box.  To move an entry up or down, use the buttons to the right of the central list box.  To modify the details of an entry, use the buttons to the left, or simply double click on the specific entry.  See Geodetic Data Catalog Entry for more information on the details associated with a catalog entry.

You can also delete an entry by selecting it and clicking the Delete this Entry button.  Add a new entry to the list by clicking the Add New Entry button.  You will be then be presented with and empty Geodetic Data Catalog Entry Dialog in which to enter the necessary information.

You can change the Fallback Datum specification by simply typing in the name of the datum definition to be used as the fallabck.  This entry must, of course, be the key name of a datum definition in the Datum Dictionary.  A Browse button is provided for your convenience.  Please note that the datum selected for a fallback must not be one which depends upon a datum shift grid file or system thereof.  It must be a datum definition which will work regardless of the geographic coordinates presented to it.  Most all datums which are used for fallbacks are of the Seven Parameter variety.

Click the Cancel button to dismiss the dialog and ignore any changes made to the currently selected catalog file.  Click OK to dismiss the dialog.  In this case, you will be asked to confirm saving any changes which you have made to the currently selected catalog file.  Of course, access to this help file is always available by clicking the Help button.

See Also

Interactive Dialogs

Coordinate System Definition Editor

Datum Editor

Ellipsoid Editor

Coordinate System Test Dialog

MGRS Test Dialog


Coordinate System Test Dialog

The Coordinate System Test dialog was originally developed to provide a means for testing newly entered coordinate system definitions.  However, the author has found it useful for all sorts of things.  Generally speaking, the Coordinate System Test dialog enables one to quickly convert a single coordinate from one coordinate system to another.

Use the upper portion of the dialog to indicate the coordinates that are to be converted and the coordinate system upon which the coordinates are based.  In the lower portion of the dialog, you will need to indicate the coordinate system to which the coordinate is to be converted.  Once the required input has been provided, clicking the calculate button will cause the indicated conversion to be performed, and the results displayed in the lower portion of the dialog box.

Next we describe the use/purpose of the several fields in the upper portion of the dialog box, i.e. that portion of the dialog that is enclosed within the group box labeled Input.

•	Coordinate System Key Name  Use this field to key in the key name of the coordinate system upon which the source coordinate is based.  You can use the browse button (the button labeled 'B' immediately to the right of the edit field) to activate the coordinate system browser.  Since there are more than a thousand coordinate system definitions in the dictionary, the browser is often helpful.  For coordinate systems which are commonly used, and the key names are therefore well known, simply keying in the name is usually the efficient approach.  Note, for state plane coordinates, you can enter the state plane zone number in lieu of the coordinate system key name.  You will need to enter a number with exactly four digits for this feature to work.

•	SP Zone Codes refer to NAD83  Check this box to indicate that any state plane code number entered in the Coordinate System Key Name field is to refer to the NAD83 version of the state plane zone.  Note that the official NAD83 version of all state plane zones are in meters rather than feet.  To indicate a NAD83 State Plane coordinate system based on, for example, U. S. Survey feet, you will need to use the key name approach.

•	Description  The description of the currently selected input coordinate system is displayed adjacent to this label.

•	Datum  The key name of the datum referenced by the selected input coordinate system is displayed adjacent to this label.

•	Source  The source of information for the selected input coordinate system definition is shown adjacent to this label.

•	Unit  The unit upon which the selected source coordinate system is based is shown adjacent to this label.

•	X/Easting/Latitude  Use this field to enter the X coordinate, the Easting, or the Latitude as may be appropriate for the coordinate system of the coordinate to be converted.  When the input coordinate system referred to in the Coordinate System Key Name field is that of a cartesian coordinate system, the label on this field will be X/Easting.  The label changes to Latitude when the referenced input coordinate system is geographic in nature.  In either case, input the appropriate numeric value.  Refer to Inputting Numbers for information as to how values may be formatted.  In this edition, the coordinate conversion system automatically determines the form of the input coordinate and processes it accordingly.

•	Y/Northing/Longitude  Use this field to enter the Y coordinate, the Northing, or the Longitude as may be appropriate for the coordinate system of the coordinate to be converted.  When the input coordinate system referred to in the Coordinate System Key Name field is that of a cartesian coordinate system, the label on this field will be Y/Northing.  The label changes to Longitude when the referenced input coordinate system is geographic in nature.  In either case, input the appropriate numeric value.  Refer to Inputting Numbers for information as to how values may be formatted.  In this edition, the coordinate conversion system automatically determines the form of the input coordinate and processes it accordingly.

•	Grid Scale  Following conversion calculation, the Coordinate System Test dialog will display in this field the grid scale factor of the input coordinate system at the indicated location.  This is a calculated field, thus user entry is not permitted.  Geographic coordinate systems do not have grid scale factors.  The concept of a single grid scale factor is compatible only with conformal map projections.  For coordinate systems based on non-conformal projections, the Coordinate System Test dialog displays a value representative of scale distortion of the projection in use at the point given.

•	Convergence Angle  Following conversion calculation, the Coordinate System Test dialog will display in this field the convergence angle of the input coordinate system at the indicated location.  The convergence angle is the difference between true north and the Y axis of the coordinate system, expressed as an azimuth.  This is a calculated field; user entry is not permitted.  The concept of a convergence angle does not apply to a geographic coordinate system.

Next we describe the use/purpose of the several fields in the lower portion of the dialog box, i.e. that portion of the dialog that is enclosed within the group box labeled Result:

•	Coordinate System Key Name  Use this field to key in the key name of the coordinate system to which the source coordinate is to be converted.  You can use the browse button (the button labeled 'B' immediately to the right of the edit field) to activate the coordinate system browser.  Since there are more than a thousand coordinate system definitions in the dictionary, the browser is often helpful.  For coordinate systems which are commonly used, and the key names are therefore well known, simply keying in the name is usually the efficient approach.  Note, for state plane coordinates, you can enter the state plane zone number in lieu of the coordinate system key name.  You will need to enter a number with exactly four digits for this feature to work.

•	SP Zone Codes refer to NAD83  Check this box to indicate that any state plane code number entered in the Coordinate System Key Name field is to refer to the NAD83 version of the state plane zone.  Note that the official NAD83 version of all state plane zones are in meters rather than feet.  To indicate a NAD83 State Plane coordinate system based on, for example, U. S. Survey feet, you will need to use the key name approach.

•	Description  The description of the currently selected result coordinate system is displayed adjacent to this label.

•	Datum  The key name of the datum referenced by the selected result coordinate system is displayed adjacent to this label.

•	Source  The source of information for the selected result coordinate system definition is shown adjacent to this label.

•	Unit  The unit upon which the selected result coordinate system is based is shown adjacent to this label.

•	X/Easting/Latitude  The Coordinate System Test dialog will display the X Coordinate, the Easting, or the Latitude (as appropriate) of the result in the edit box underneath this label.  This field is for calculated results; user input is not required nor is it permitted.  The results can, however, be cut for pasting in another field, window, or application.  When the result coordinate system referred to in the Coordinate System Key Name field is that of a cartesian coordinate system, the label on this field will be X/Easting.  The label changes to Latitude when the referenced input coordinate system is geographic in nature.  In the case of a latitude, degrees, minutes, and seconds form is used by default.  Check the decimal degrees box at the bottom of the dialog to obtain values in decimal degree form.

•	Y/Northing/Longitude  The Coordinate System Test dialog will display the Y Coordinate, the Northing, or the Longitude (as appropriate) of the result in the edit box underneath this label.  This field is for calculated results; user input is not required nor is it permitted.  When the result coordinate system referred to in the Coordinate System Key Name field is that of a cartesian coordinate system, the label on this field will be Y/Northing.  The label changes to Longitude when the referenced input coordinate system is geographic in nature.  In the case of a longitude, degrees, minutes, and seconds form is used by default.  Check the decimal degrees box at the bottom of the dialog to obtain values in decimal degree form.

•	Grid Scale  Following conversion calculation, the Coordinate System Test dialog will display in this field the grid scale factor of the result coordinate system at the calculated location.  Geographic coordinate systems do not have grid scale factors.  The concept of a single grid scale factor is compatible only with conformal map projections.  For coordinate systems based on non-conformal projections, the Coordinate System Test dialog displays a value representative of scale distortion of the projection in use at the calculated location.

•	Convergence Angle  Following conversion calculation, the Coordinate System Test dialog will display in this field the convergence angle of the result coordinate system at the calculated location.  The convergence angle is the difference between true north and the Y axis of the coordinate system, expressed as an azimuth.  Note that this is a calculated field and user entry is not permitted, and that the concept of a convergence angle does apply to a geographic coordinate system.

Underneath the Result group box, lie the Decimal Degrees, High Precision, and 3D Calculation check boxes.  Checking the Decimal Degrees box will cause all latitude, longitude, and convergence angle output to be made in decimal degree format (rather than degrees, minutes, and seconds).  Check the High Precision box to have resulting values reported in high precision as opposed to normal precision.  Check the 3D Calculation box to cause three dimensional datum conversions performed.

See Also

Interactive Dialogs

Coordinate System Definition Editor

Datum Editor

Ellipsoid Editor

Geodetic Data Catalog Editor

MGRS Test Dialog


MGRS Test Dialog

Use the Military Grid Reference System (MGRS) Test Dialog to convert coordinates from MGRS to UTM/geographic coordinates and vice versa.   While originally designed as a means to test the Coordinate Conversion System's implementation of MGRS, it can also be a useful tool when small numbers of coordinates need to be converted.  This dialog will also provide for the proper calculation of UTM zones, including the polar regions, and the strange stuff which goes on in Northern Europe (i.e. Norway and the Svaldberg Islands).

The MGRS test dialog is grouped into four sections; programmers use the term group boxes to refer to these sections.  The first group box, labeled Ellipsoid, shows the descriptive name of the currently selected ellipsoid.  That is, the ellipsoid upon which all of the calculations are to be based.  Use the Select button to choose a different ellipsoid for the calculations.

The remainder of the dialog consists of three different group boxes, one for each of the three different types of coordinates that are manipulated by the test program.  These three coordinate types are Military Grid Reference System designations, Latitude/Longitude (i.e. geographic coordinates), and UTM/UPS coordinates with zone number.  (UPS is an acronym for Universal Polar Stereographic, the system that applies to the polar regions.)  Consider each of these three areas as an input area.  Use the Military Grid Reference System section to input an MGRS designation.  When input is complete, click the Enter button in that group box to cause the input to be processed and the calculation results displayed in the remaining two boxes.

In a similar manner, you can input data in any of the three group boxes and click the associated Enter button to cause the remaining group boxes to be filled in with the correct results.  Thus, a single dialog can be used to convert from any one system to any other.  Please be sure to click the Enter box associated with the data you just entered.  This is how the program knows which system of coordinates are to be used as input, and what results are to be produced.

Within the MGRS group box, the dialog provides a means by which you can adjust the precision of the MGRS designation computed.  This value can range from 0 to a maximum of 5.  Note that, at a precision of 5, the MGRS designation refers to a specific square meter on the surface of the earth.  The resulting geographic and UTM coordinate calculations are based on the specific point at the center of this square.  A precision of 4 would produce the MGRS designation for a 10 meter by 10 meter square.  Also in the MGRS group box, you will find a check box labeled Use Bessel Codes.  The definition of MGRS suggests that an alternative encoding system should be used for certain ellipsoids, specifically the Bessel as used in Western Europe.  Check this box to have the MGRS test dialog use the alternative encoding system.

In the Latitude and Longitude group box, you can enter (and the dialog will display results) in the form of latitude and longitude coordinates.  You may enter the coordinates in decimal form, or using degrees minutes and seconds form as outlined elsewhere in this Help File/document.  Refer to Inputting Numbers for information as to how values may be formatted.    The output format is always in degrees minutes, and seconds form.

In the UTM/UPS group box, you may enter UTM or UTM coordinates.  When entering coordinates, be sure to select the proper zone.  Note that North and South are included in the zone combo box and should be used to refer to the north and south polar regions respectively when appropriate.  Of course, the MGRS test dialog will use this area to return to you the results of conversions.

Use the Close button near the bottom of the dialog to close the dialog box when finished using it.  Use the adjacent Help button to gain access to this help file.

See Also

Interactive Dialogs

Coordinate System Definition Editor

Datum Editor

Ellipsoid Editor

Geodetic Data Catalog Editor

Coordinate System Test Dialog


Supported Projections

Currently, 36 different projections are supported.  Variations to several of these and 2 pseudo projections are also supported.  Points of interest for each of these projections and all variations thereof are given in this section.  New projections are routinely added to the product, so be sure to contact your software vendor for the latest information.

The order in which the various projections and their variations are presented is admittedly haphazard; mostly to do with the order in which they were implemented.  The index will be useful to locate the particular information you need.

In This Section  Null Projection, produces/processes Latitude & Longitude  Lambert Conformal Conic Projection, Two Standard Parallels  Lambert Conformal Conic Projection, One Standard Parallel  Lambert Conformal Conic Projection, Belgian Variation  Lambert Conformal Conic, Wisconsin County Variation  Lambert Conformal Conic, Minnesota DOT Variation  Transverse Mercator or Gauss Kruger Projection  Transverse Mercator with Affine Post Process  Universal Transverse Mercator System  South Oriented Transverse Mercator  Transverse Mercator, Wisconsin County Variation  Transverse Mercator, Minnesota DOT Variation  Transverse Mercator, ala Snyder  Albers Equal Area Conic Projection  Unrectified Hotine Oblique Mercator Projection  Alaska Variation of Hotine Oblique Mercator Projection  Unrectified Hotine Oblique Mercator Projection, Two Point Form  Rectified Skew Orthomorphic, Azimuth at Projection Center  Rectified Skew Orthomorphic, Skew Azimuth at Rectified Origin   Rectified Skew Orthomorphic, Origin & Azimuth at Center  Rectified Skew Orthomorphic, Two Point Form  Mercator Cylindrical Projection  Lambert Azimuthal Equidistant Projection  Lambert Azimuthal Equidistant, Elevated Ellipsoid  Lambert Azimuthal Equal Area Projection   Lambert Tangential Conformal Conic Projection  Hassler American Polyconic Projection  Lallemand IMW Modified Polyconic Projection  Equidistant Conic Projection, a.k.a. Simple Conic  Miller Cylindrical Projection  Oblique Stereographic Projection, per Snyder (USA)  Oblique Stereographic Projection  Polar Stereographic Projection  Sinusoidal Projection  Orthographic Projection  Gnomonic Projection  Equidistant Cylindrical Projection  Van Der Grinten Projection  Cassini Projection  Modified Stereographic Projection  New Zealand National Grid  Robinson Cylindrical Projection  Bonne Pseudoconical Projection  Ekert Pseudocylindrical Projection, Number IV  Ekert Pseudocylindrical Projection, Number VI  Mollweide Projection  Goode Homolosine Projection  Normal Aspect, Equal Area Cylindrical Projection  Transverse Aspect, Equal Area Cylindrical Projection  Bipolar Oblique Conformal Conic Projection  Swiss Oblique Cylindrical Projection  Krovak Oblique Conformal Conic, Czechoslovkia  Krovak Oblique Conformal Conic, Czechoslovkia  Non-georeferenced (aka non-earth) coordinate system  Oblique Cylindrical Projection (Generalized)  Danish System 34, UTM + polynomials (pre-1999)  Danish System 34, UTM + polynomials (1999 vintage)  Ordnance Survey National Grid Transformation of 1997  Ordnance Survey National Grid Transformation of 2002	See Also  Coordinate Conversion System  Overview  Coordinate System Referencing  Coordinate System Key Name Conventions  Dictionary Entry Protection  Interactive Dialog Support  Interactive Dialogs  Geodetic Reference  Geodetic Data Catalog


Null Projection, produces/processes Latitude & Longitude

This is not really a projection in the true sense of the word.  We refer to this as a pseudo projection.  This pseudo projection does very little and is a design trick that enables the Coordinate Conversion System to process geographic coordinates in exactly the same manner as any cartesian coordinate.  Thus, the user interface does not require any special provisions in order to handle geographic coordinates.

An additional benefit of this technique is that definitions of geographic coordinate systems also appear in the Coordinate System Dictionary.  Therefore, each such definition can be, and is, referenced to either a datum or ellipsoid (e.g. LL27, LL83, LL84).  Also, each geographic coordinate system can be, and is, assigned a specific system unit (e.g. degrees, grads, minutes).

This projection, and therefore all geographic coordinate systems, recognizes the origin longitude field of the coordinate system definition and uses it as the prime meridian for the geographic coordinate system.  Thus, setting the Origin Longitude field to 2 20' 14.025"E will reference a geographic coordinate system to the Meridian of Paris.

This projection also accepts two parameters labeled Western End of Longitude Range and Eastern End of Longitude Range.  You can use these parameters to define, for example, a geographic coordinate system in which longitudes are reported in the range of 0 through 360.  Leaving these parameters set to zero will cause the Coordinate Conversion System to default the range to the normal -180 (west longitude) through +180 (east longitude).


Lambert Conformal Conic Projection, Two Standard Parallels

This is the basic form of the widely used Lambert Conformal Conic Projection.  In addition to an origin longitude and origin latitude, it requires two parameters referred to as Standard Parallels which are used to produce the shrinkage affect which is common in map projection usage.  The Coordinate Conversion System labels these parameters as Northern Standard Parallel and Southern Standard Parallel but the north/south relationship is unimportant.  That is why similar products that perform  coordinate conversion function will usually refer to these as Standard Parallel 1 and Standard Parallel 2.

Note that the standard parallels can be on opposite sides of the equator, but only if they are not equidistant from the equator.  (If the standard parallels were equidistant, the result would be cylindrical, not conic.)

By setting the both standard parallel parameters to precisely the same value, you can obtain a single parallel variation of this projection.  This result will differ slightly from the variation next described in that the resulting variation can have an origin latitude which is different from that of the standard parallel.  Refer to the distribution coordinate system definition for SAMOA for an example of this usage.  Note that this usage precludes the use of the shrinkage affect.


Lambert Conformal Conic Projection, One Standard Parallel

This form of the Lambert is rarely used in North America, but is frequently used elsewhere, especially in Europe.  In this case, the standard parallel and the Origin Latitude are the same, and a Scale Reduction factor is used to obtain the shrinkage affect.

Since the Origin Latitude, as distinct from the standard parallel, has no affect on the magnitude or distribution of projection distortion, the lack of an origin latitude distinct from the standard parallel can be overcome through the use of an appropriate value for the False Northing.


Lambert Conformal Conic Projection, Belgian Variation

This is a variation of the Two Standard Parallel version of the Lambert which is used only in Belgium.  Parameter usage is identical to the Two Standard Parallel variation described above.


Lambert Conformal Conic, Wisconsin County Variation

This is a variation of the Two Standard Parallel version of the Lambert Conformal Conic that is used to define coordinate systems for many of the individual counties in the state of Wisconsin.

This variation requires two additional parameters that are used to expand the referenced ellipsoid to account for the average elevation in the subject county.  The first parameter is the Average Geoid Height of the county specified in meters.  The second parameter is the Average Elevation (i.e. elevation above the geoid or orthometric height) of the subject county given in system units.


Lambert Conformal Conic, Minnesota DOT Variation

This is a variation of the Two Standard Parallel version of the Lambert Conformal Conic that is used to define coordinate systems for several of the individual counties in the state of Minnesota.

This variation requires an additional parameter that is used to expand the referenced ellipsoid to account for the average elevation in the subject county.  This parameter is Average Elevation (i.e. elevation above the geoid or orthometric height) of the subject county given in system units.


Transverse Mercator or Gauss Kruger Projection

This is the Coordinate Conversion System's implementation of the ubiquitous Transverse Mercator projection, also known as the Gauss Kruger Projection, or more simply the Gauss projection.  This projection is used world wide and is the projection used in the Universal Transverse Mercator (UTM) system of coordinate systems.

This projection requires an Origin Latitude and an origin longitude.  However, due to historical reasons, the origin longitude is specified as a parameter on the Parameter Page labeled Central Meridian.   A Scale Reduction factor is also required.  Outside of North America, it is not uncommon for the scale reduction factor to be set to unity (i.e. 1.0), thus essentially disabling this feature.


Transverse Mercator with Affine Post Process

This is yet another variation of the popular Transverse Mercator or Gauss Kruger Projection.  This variation includes the application of an affine transformation to the results produced by the normal Transverse Mercator projection calculation.  This variation was added to support local coordinate systems in Sweden.  Perhaps it will be useful elsewhere as well.

The Affine Transformation is implemented as follows, and is applied to the horizontal cartesian coordinates which result from the normal projection algorithm:

Xnew = A0 + A1xold + A2yold

Ynew = B0 + B1xold + B2yold

As you can see, this implementation supports the use of six parameters, all six of which can be specified by the user.  The six parameters are:

Parameter	Parameter Number	Parameter Name
A0	2	Affine Transformation A0 Coefficient
B0	3	Affine Transformation A0 Coefficient
A1	4	Affine Transformation A0 Coefficient
A2	5	Affine Transformation A0 Coefficient
B1	6	Affine Transformation A0 Coefficient
B2	7	Affine Transformation A0 Coefficient
Note that in order for the inverse to function properly, the following condition is not allowed:

A1B2 = A2B1

Also note, that by setting B2 equal to A1, and B1 equal to the complement of A2 one can obtain the Helmert, or Similarity Transformation.


Universal Transverse Mercator System

This variation is simply an interface to the Transverse Mercator projection described immediately above.   It differs only in that the parameters required are UTM Zone Number and Hemisphere.  Given these two simple parameters, this variation will automatically calculate the origin latitude, central meridian, scale reduction, false easting, and false northing parameters as appropriate for the indicated UTM zone and the selected units.

As the value for the Hemisphere parameter, use a positive value, like 1.0 to indicate the northern hemisphere and a negative value, like  -1.0, to indicate the southern hemisphere.

Note that you will still need to reference the coordinate system definition to the desired datum or ellipsoid, and select the desired system unit.


South Oriented Transverse Mercator

This is the variation of the Transverse Mercator Projection used in South Africa.  The required parameters are the same as the standard Transverse Mercator.


Transverse Mercator, Wisconsin County Variation

This is a variation of the standard Transverse Mercator that is used to define coordinate systems for several of the individual counties in the state of Wisconsin.

This variation requires two additional parameters that are used to expand the referenced ellipsoid to account for the average elevation in the subject county.  The first parameter is the Average Geoid Height of the county specified in meters.  The second is the Average Elevation (i.e. elevation above the geoid or orthometric height) of the subject county given in system units.


Transverse Mercator, Minnesota DOT Variation

This is a variation of the standard Transverse Mercator that is used to define coordinate systems for many of the individual counties in the state of Minnesota.

This variation requires an additional parameter That is used to expand the referenced ellipsoid to account for the average elevation in the subject county.  This parameter is Average Elevation (i.e. elevation above the geoid or orthometric height) of the subject county given in system units.

Note, this is a rigorous implementation of the Minnesota variation.  The approximations of scale reduction and false easting and northing frequently used in other products should not be used here.


Transverse Mercator, ala Snyder

This is a variation of the standard Transverse Mercator that is based on the formulations published by John Parr Snyder in Map Projections - A Working Manual.  The developers of the Coordinate Conversion System used this formulation exclusively in early releases.   The Coordinate Conversion System now includes a newer, faster, more accurate formulation, and makes this new implementation available as its standard Transverse Mercator projection.  However, there remain clients who prefer the comfort of coordinate values which match precisely the values they are used to.  Therefore. the Snyder formulation is preserved as a part of the Coordinate Conversion System's heritage and is assigned this particular name.

Parameter usage of this projection is the same as the standard Transverse Mercator described above.


Albers Equal Area Conic Projection

In addition to an Origin Longitude and Origin Latitude, the Albers Equal Area Projection requires two parameters referred to as Standard Parallels that are used to produce the shrinkage affect which is common in map projection usage.  The Coordinate Conversion System labels these parameters as Northern Standard Parallel and Southern Standard Parallel but the north/south relationship is unimportant.  That is why similar products that perform coordinate conversion will refer to these as Standard Parallel 1 and Standard Parallel 2.

Note that the standard parallels can be on opposite sides of the equator, but only if they are not equidistant from the equator.  (If the standard parallels were equidistant, the result would be cylindrical, not conic.)

By setting the both standard parallel parameters to precisely the same value, you can obtain a single parallel variation of this projection.

This projection is that which is used by the US Geological Survey to produce 1:2,000,000 Digital Line Graph data files for the 48 conterminous states, Alaska, and Hawaii.


Unrectified Hotine Oblique Mercator Projection

This is one of many variations of a class of projections that is often referred to as Oblique Mercator.  The formulas developed by Hotine are commonly used, but formulas have also been developed by Rosenmund and Laborde.  Snyder has also published a variation of Hotine's formulas.  All of these formulations produce similar, but not precisely identical, results.  Thus, it is not uncommon to qualify the general Oblique Mercator with some indication of the formulation used.

Unrectified, in this case, means that the axis of the X coordinate is coincident with the central geodesic of the projection, implying that the axis of the Y coordinate is not true north as it is with most all other projections.  In the literature, these unrectified axes are usually referred to as the u and v axes respectively.

This projection requires three parameters.  The Central Point Latitude and Central Point Longitude parameters define the approximate center of the region to be mapped, and the Geodesic Azimuth at Projection Center parameter defines the azimuth of the central geodesic at the central point in degrees east of north.  The true origin of this projection is at the point where "the central geodesic intersects the equator of the 'aposphere,' and is, thus, a rather academic location" (Snyder, p70).  This makes this projection rather difficult to use and we know of no actual use of this projection in its unrectified form.


Alaska Variation of Hotine Oblique Mercator Projection

This is one of many variations of a class of projections that is often referred to as Oblique Mercator.  The formulas developed by Hotine are commonly used, but formulas have also been developed by Rosenmund and Laborde.  Snyder has also published a variation of Hotine's formulas.  All of these formulations produce similar, but not precisely identical, results.  Thus, it is not uncommon to qualify the general Oblique Mercator with some indication of the formulation used.

This variation is of the Hotine Oblique Mercator is the version published by Snyder and is the projection used for Zone 1 of the Alaska State Plane Coordinate System.  Despite much published information to the contrary, it is not the same as the Rectified Skew Orthomorphic projection used elsewhere.  We suspect, but do not know for a fact, that this projection is also used for the Great Lakes Survey.  This projection is rectified, thus the Y axis is indeed true north (at the origin anyway).

This variation requires three parameters.  The Central Point Latitude and Central Point Longitude parameters define the approximate center of the region to be mapped, and the Geodesic Azimuth at Projection Center parameter defines the azimuth of the central geodesic at the central point in degrees east of north.  The true origin of this projection is at the point where "the central geodesic intersects the equator of the 'aposphere,' and is, thus, a rather academic location" (Snyder, p70).  This makes this projection rather difficult for us mere mortals to create a new coordinate system using it.


Unrectified Hotine Oblique Mercator Projection, Two Point Form

This is one of many variations of a class of projections that is often referred to as Oblique Mercator.  The formulas developed by Hotine are commonly used, but formulas have also been developed by Rosenmund and Laborde.  Snyder has also published a variation of Hotine's formulas.  All of these formulations produce similar, but not precisely identical, results. Thus, it is not uncommon to qualify the general Oblique Mercator with some indication of the formulation used.

Unrectified, in this case, means that the axis of the X coordinate is coincident with the central geodesic of the projection, implying that the axis of the Y coordinate is not true north as it is with most all other projections. In the literature, these unrectified axes are usually referred to as the U and V axes respectively.

This variation differs from the Unrectified Hotine Oblique Mercator described above only in the manner in which the central geodesic is defined.  This variation requires an origin latitude and two points to define the location and orientation of the central geodesic.  The Coordinate Conversion System, perhaps erroneously, uses its Origin Latitude parameter as what is actually the Central Latitude.  That is, the projection center is defined as the point at which the central geodesic crosses the central latitude.  (Just in case your not thoroughly confused already, consider that there are two such locations.)

Again, the true origin of this projection is at the point where "the central geodesic intersects the equator of the 'aposphere,' and is, thus, a rather academic location" (Snyder, p70).  This makes this projection rather difficult to use and, fortunately, we know of no actual application of this projection in this form.


Rectified Skew Orthomorphic, Azimuth at Projection Center

This is one of many variations of a class of projections that is often referred to as Oblique Mercator.  The formulas developed by Hotine are commonly used, but formulas have also been developed by Rosenmund and Laborde.  Snyder has also published a variation of Hotine's formulas. All of these formulations produce similar, but not precisely identical, results.  Thus, it is not uncommon to qualify the general Oblique Mercator with some indication of the formulation used.

This is the formulation/variation of the Oblique Mercator which most widely used (Alaska excepted).  This variation requires three parameters.  The Central Point Latitude and Central Point Longitude parameters define the approximate center of the region to be mapped, and the Geodesic Azimuth at Projection Center parameter defines the azimuth of the central geodesic at the central point in degrees east of north.  (This is the point given by the first two parameters.)

The true origin of this projection is at the point where "the central geodesic intersects the equator of the 'aposphere,' and is, thus, a rather academic location" (Snyder, p70).  Thus, neither the Origin Latitude or Origin Longitude parameter are used.  This also makes this projection rather difficult for use when defining a new custom coordinate system, see Rectified Skew Orthomorphic, Origin at Center below.

Note that another variation of this projection, named Rectified Skew Orthomorphic, Skew Azimuth at Rectified Origin, supports an azimuth parameter referred to as: Skew Azimuth at Projection Origin.  I t might seem to the casual cartographer that these would be the same; but on an ellipsoid, these numbers are different by a small (but significant) amount.


Rectified Skew Orthomorphic, Skew Azimuth at Rectified Origin

This is one of many variations of a class of projections that is often referred to as Oblique Mercator.  The formulas developed by Hotine are commonly used, but formulas have also been developed by Rosenmund and Laborde.  Snyder has also published a variation of Hotine's formulas. All of these formulations produce similar, but not precisely identical, results.  Thus, it is not uncommon to qualify the general Oblique Mercator with some indication of the formulation used.

This is the formulation/variation of the Oblique Mercator which most widely used (Alaska excepted).  This variation requires three parameters.  The Central Point Latitude and Central Point Longitude parameters define the approximate center of the region to be mapped, and the Skew Azimuth at Rectified Origin parameter defines the azimuth of the central geodesic at the true origin of the projection as described below.  This is also the origin of the rectified coordinate system.  This parameter is also referred to as the Skew Azimuth in other applications and books on coordinate conversion.  In any case, this azimuth is given in degrees east of north.  That is, use a value greater than 180 or a negative value for an azimuth which is west of north.

The true origin of this projection is at the point where "the central geodesic intersects the equator of the 'aposphere,' and is, thus, a rather academic location" (Snyder, p70).  Thus, neither the Origin Latitude or Origin Longitude parameter are used.  This also makes this projection rather difficult for use when defining a new custom coordinate system, see Rectified Skew Orthomorphic, Origin at Center below.

Note that another variation of this projection, named Rectified Skew Orthomorphic, Azimuth at Projection Center, supports an azimuth parameter referred to as: Geodesic Azimuth at Projection Center.  It might seem to the casual cartographer that these would be the same; but on an ellipsoid, these numbers are different by a small (but significant) amount.

See Also

Supported Projections

Null Projection, produces/processes Latitude & Longitude

Lambert Conformal Conic Projection, Two Standard Parallels

Lambert Conformal Conic Projection, One Standard Parallel

Lambert Conformal Conic Projection, Belgian Variation

Lambert Conformal Conic, Wisconsin County Variation

Lambert Conformal Conic, Minnesota DOT Variation

Transverse Mercator or Gauss Kruger Projection

Transverse Mercator with Affine Post Process

Universal Transverse Mercator System

South Oriented Transverse Mercator

Transverse Mercator, Wisconsin County Variation

Transverse Mercator, Minnesota DOT Variation

Transverse Mercator, ala Snyder

Albers Equal Area Conic Projection

Unrectified Hotine Oblique Mercator Projection

Alaska Variation of Hotine Oblique Mercator Projection

Unrectified Hotine Oblique Mercator Projection, Two Point Form

Rectified Skew Orthomorphic, Azimuth at Projection Center

Rectified Skew Orthomorphic, Origin & Azimuth at Center

Rectified Skew Orthomorphic, Two Point Form

Mercator Cylindrical Projection

Lambert Azimuthal Equidistant Projection

Lambert Azimuthal Equidistant, Elevated Ellipsoid

Lambert Azimuthal Equal Area Projection

Lambert Tangential Conformal Conic Projection

Hassler American Polyconic Projection

Lallemand IMW Modified Polyconic Projection

Equidistant Conic Projection, a.k.a. Simple Conic

Miller Cylindrical Projection

Oblique Stereographic Projection, per Snyder (USA)

Oblique Stereographic Projection

Polar Stereographic Projection

Sinusoidal Projection

Orthographic Projection

Gnomonic Projection

Equidistant Cylindrical Projection

Van Der Grinten Projection

Cassini Projection

Modified Stereographic Projection

New Zealand National Grid

Robinson Cylindrical Projection

Bonne Pseudoconical Projection

Ekert Pseudocylindrical Projection, Number IV

Ekert Pseudocylindrical Projection, Number VI

Mollweide Projection

Goode Homolosine Projection

Normal Aspect, Equal Area Cylindrical Projection

Transverse Aspect, Equal Area Cylindrical Projection

Bipolar Oblique Conformal Conic Projection

Swiss Oblique Cylindrical Projection

Krovak Oblique Conformal Conic, Czechoslovkia

Krovak Oblique Conformal Conic, Czechoslovkia

Non-georeferenced (aka non-earth) coordinate system

Oblique Cylindrical Projection (Generalized)

Danish System 34, UTM + polynomials (pre-1999)

Danish System 34, UTM + polynomials (1999 vintage)

Ordnance Survey National Grid Transformation of 1997

Ordnance Survey National Grid Transformation of 2002


Rectified Skew Orthomorphic, Origin & Azimuth at Center

This is one of many variations of a class of projections that is often referred to as Oblique Mercator.  The formulas developed by Hotine are commonly used, but formulas have also been developed by Rosenmund and Laborde.  Snyder has also published a variation of Hotine's formulas. All of these formulations produce similar, but not precisely identical, results.  Thus, it is not uncommon to qualify the general Oblique Mercator with some indication of the formulation used.

While this formulation of the Oblique Mercator is that which is most widely used, this specific variation has rarely been used in the past.  This variation includes provisions to automatically adjust the origin of the projection to appear to be at the central point defined by the parameters next described.  Thus, this is the preferred Oblique Mercator variation for the definition of new custom coordinate systems.

This variation requires three parameters.  The Central Point Latitude and Central Point Longitude parameters define the approximate center of the region to be mapped, and the Geodesic Azimuth at Projection Center parameter defines the azimuth of the central geodesic at the central point in degrees east of north.

The azimuth parameter is the azimuth at the projection center, i.e. the point given by the first two parameters. It is not what is often called the Skew Azimuth, which is the azimuth of the central geodesic at the rectified origin.  Remember, it is the rather strange origin which makes this specific variation valuable.


Rectified Skew Orthomorphic, Two Point Form

This is one of many variations of a class of projections that is often referred to as Oblique Mercator.  The formulas developed by Hotine are commonly used, but formulas have also been developed by Rosenmund and Laborde.  Snyder has also published a variation of Hotine's formulas.  All of these formulations produce similar, but not precisely identical, results.  Thus, it is not uncommon to qualify the general Oblique Mercator with some indication of the formulation used.

This variation uses two points to define the Central Geodesic and uses the Origin Latitude (erroneously) to specify the central latitude; the approximate center of the region being mapped being at the point where the central geodesic crosses the central latitude.


Mercator Cylindrical Projection

In years past, this projection has been used to produce the standard schoolroom map of the world.  It remains a popular choice for marine navigation charts as rhumb lines are straight.   This projection uses a default origin latitude of zero (i.e. the equator) and requires that you specify a Central Meridian (i.e. origin longitude) on the Parameters Page.  Additionally, a Standard Parallel may be specified to achieve the commonly used shrinkage affect.


Lambert Azimuthal Equidistant Projection

Traditional implementations of this projection only require the Origin Latitude and Origin Longitude parameters.

However, the coordinate conversion system's implementation adds an additional parameter that is, perhaps, unique to it.  You may specify a Y Axis Azimuth parameter to indicate the azimuth of the Y axis of the coordinate system relative to true north.  Specify this value in degrees east of north.  That is, use a positive value for a Y axis which is east of true north, and a negative value in the case of a Y axis which is west of true north.  This additional parameter is useful when this projection is used to georeference a local grid system (a.k.a. company coordinate system).  Set this value to zero for coordinate systems where the Y axis is coincident with true north.  In this case, this projection produces the same results as any of the more traditional implementations.


Lambert Azimuthal Equidistant, Elevated Ellipsoid

Traditional implementations of the Lambert Azimuthal Equidistant Projection only require the Origin Latitude and Origin Longitude parameters.  However, the coordinate conversion system's implementation of this projection adds two additional parameters that are, perhaps, unique to it.

Use the Y Axis Azimuth parameter to indicate the azimuth of the Y axis of the coordinate system relative to true north.  Specify this value in degrees east of north.  That is, use a positive value for a Y axis which is east of true north, and a negative value in the case of a Y axis which is west of true north.  This additional parameter is useful when this projection is used to georeference a local grid system (a.k.a. company coordinate system).  Set this value to zero for coordinate systems where the Y axis is coincident with true north.

Use the Average Elevation parameter to indicate the average elevation of the region covered by the coordinate system.  This value should actually be ellipsoidal height, and is always given in the linlear units of the coordinate system.  That is, if the linear units of the easting and northing (X and Y) values of this coordinate system are in U. S. Survey Feet, than the ellipsoid height must also be given in U. S. Survey Feet.

The Coordinate Conversion System will use this ellipsoid elevation to correct the linear distance (i.e. X's and Y's) to angular distance (i.e. latitudes and longitudes) calculations to account for the elevation of the coordinate system.  This is also an advantage when using this projection to georeference a local grid system (a.k.a. company coordinate system).  Setting this value to zero will eliminate the effect of this parameter, and result in the traditional calculation of this projection.


Lambert Azimuthal Equal Area Projection

This projection requires only the Origin Latitude and Origin Longitude parameters.

However, the Coordinate Conversion System's implementation adds an additional parameter that is, perhaps, unique to it.  You may specify a Y Axis Azimuth parameter to indicate the azimuth of the Y axis of the coordinate system relative to true north.  Specify this value in degrees east of north.  This additional parameter is useful when this projection is used to georeference a local grid system (a.k.a. company coordinate system).  Of course, setting this parameter to zero will produce a coordinate system equivalent to that of the traditional Azimuthal Equal Area.


Lambert Tangential Conformal Conic Projection

This projection is an alternative formulation of the Lambert Conformal Conic, One Standard Parallel variation. The formulation used here is that published by the National Geographic Institute of France and may produce slightly more precise results at the expense of calculation speed.

This projection requires an Origin Latitude (which also serves as the single standard parallel), an Origin Longitude, and a Scale Reduction Factor.


Hassler American Polyconic Projection

This is the Polyconic Projection developed by Ferdinand Rudolph Hassler for use in mapping North America.  It is significantly different from the Modified Polyconic developed by Lallemand for the production of the International Map of the World.  So much different, that we would not call either one a variation, or alternate formulation, of the other.

This projection requires an Origin Latitude and a Central Meridian which serves as an origin longitude.  For historical reasons, and perhaps erroneously, the Coordinate Conversion System presents the Central Meridian as a parameter on the Parameter Page and disables the origin longitude field.


Lallemand IMW Modified Polyconic Projection

This is the Polyconic Projection developed by Lallemand for the production of the International Map of the World.  It is significantly different from the Hassler American Polyconic developed by Ferdinand Rudolph Hassler for use in mapping North America.  So much different, that we would not call either one a variation, or alternate formulation, of the other.

This projection requires four parameters all of which are specified on the Parameter page. The Central Meridian defines the origin longitude.  The Eastern Meridian parameter defines a meridian of true scale some distance to the east of the central meridian and implies an equivalent true scale meridian the same distance to the west of the central meridian.  The Northern Parallel and Southern Parallel define the north south extents of the map.

Unlike most other projections, the Lallemand Modified Polyconic runs into serious mathematical trouble when processing coordinates significantly outside the region intended to be mapped.


Equidistant Conic Projection, a.k.a. Simple Conic

This projection requires an Origin Longitude, an Origin Latitude, and two parameters referred to as Standard Parallels which are used to produce the shrinkage affect which is common in map projection usage.  The Coordinate Conversion System labels these parameters as Northern Standard Parallel and Southern Standard Parallel but the north/south relationship is unimportant.  That is why similar products that perform coordinate conversion will usually refer to these as Standard Parallel 1 and Standard Parallel 2.

Note that the standard parallels can be on opposite sides of the equator, but only if they are not equidistant from the equator.  (If the standard parallels were equidistant, the result would be cylindrical, not conic.)

By setting the both standard parallel parameters to precisely the same value, you can obtain a single parallel variation of this projection.


Miller Cylindrical Projection

This projection has become a popular alternative to the Mercator for making the standard school room map of the world.  It presumes an origin latitude of the equator.  The Central Meridian parameter must be used to specify the origin longitude.  Again, this can be considered erroneous and is a historical artifact of the Coordinate Conversion System's development over the years.


Oblique Stereographic Projection, per Snyder (USA)

There are two formulations of the Oblique Stereographic.  This formulation was developed by Snyder and has been supported by the Coordinate Conversion System for many years now.  However, it is not the formulation commonly used in Europe and other regions.  Which formulation is better has been the topic of many interesting discussions.  In general, use this formulation to match results produced by previous versions of any product that uses the Coordinate Conversion System.  Use the formulation next described to achieve the results expected in the Netherlands, New Brunswick, and elsewhere.

This projection requires an Origin Latitude, and Origin Longitude, and a Scale Reduction Factor.

The Coordinate Conversion System's implementation of this projection adds an additional parameter that is, perhaps, unique to it.  You may specify a Y Axis Azimuth parameter to indicate the azimuth of the Y axis of the coordinate system relative to true north.  Specify this value in degrees east of north.  This additional parameter is useful when this projection is used to georeference a local grid system (a.k.a. company coordinate system).  Of course, setting this parameter to zero will produce a coordinate system equivalent to that of the traditional Stereographic ala Snyder.


Oblique Stereographic Projection

This formulation of the Oblique Stereographic is that which is commonly used outside of the U. S., specifically in the Netherlands, Romania, and New Brunswick, Canada.  We have seen references to this formulation as the RD Stereographic and Double Stereographic as well.

This projection requires an Origin Latitude, and Origin Longitude, and a Scale Reduction Factor.


Polar Stereographic Projection

For the Polar aspect of the Stereographic, we know of only one formulation.  For this specific aspect, this formulation is the same as that supported by previous releases of the Coordinate Conversion System.

This projection requires an Origin Latitude, and Origin Longitude, and a Scale Reduction Factor.  Of course, the origin latitude must be either +90 degrees or -90 degrees.  Any other value will produce an error condition.


Sinusoidal Projection

This projection is often used to map large regions of the earth, often in interrupted form.  Currently, the Coordinate Conversion System does not support the interrupted form of this projection, but such support is expected soon.

This projection presumes an origin latitude of the equator and requires that you specify the Origin Longitude which will become the center of any map based on the resulting coordinate system.


Orthographic Projection

This projection produces globe like maps of a single hemisphere.  The Coordinate Conversion System supports polar, equatorial, and oblique aspects of this projection.  It requires specification of an Origin Latitude and Origin Longitude.

Note that this projection is implemented only for the sphere, and is usually cartographically referenced directly to an ellipsoid.  If referenced to an ellipsoid (rather than a sphere), the Coordinate Conversion System will use the equatorial radius of the ellipsoid as the radius of the sphere.


Gnomonic Projection

This projection produces globe like maps of a single hemisphere.  The Coordinate Conversion System supports polar, equatorial, and oblique aspects of this projection.  It requires specification of an Origin Latitude and Origin Longitude.

Note that this projection is implemented only for the sphere, and is usually cartographically referenced directly to an ellipsoid.  If referenced to an ellipsoid (rather than a sphere), the Coordinate Conversion System will use the equatorial radius of the ellipsoid as the radius of the sphere.


Equidistant Cylindrical Projection

This projection is the simplest of all projections.  It requires the specification of an Origin Latitude, Origin Longitude, and a Standard Parallel.

Setting the origin latitude and standard parallel values to zero has the effect of creating the equivalent of the Platte Carrée projection.

Note that this projection is implemented only for the sphere, and is usually cartographically referenced directly to an ellipsoid.  If referenced to an ellipsoid (rather than a sphere), the Coordinate Conversion System will use the equatorial radius of the ellipsoid as the radius of the sphere.  When used to obtain the Platte Carrée variation, a sphere with a radius of 57.29577951 (i.e. one radian expressed in degrees) is often used.


Van Der Grinten Projection

This is yet another projection used to produce the traditional schoolroom map of the world.  It presumes an origin latitude of zero and requires that you provide an Origin Longitude which becomes the center of maps produced using the resulting coordinate system.

Note that this projection is implemented only for the sphere, and is usually cartographically referenced directly to an ellipsoid.  If referenced to an ellipsoid (rather than a sphere), the Coordinate Conversion System will use the equatorial radius of the ellipsoid as the radius of the sphere.


Cassini Projection

Also known as the Cassini-Soldner, this projection is the forerunner of the Transverse Mercator and shares many of its properties.  It has been in use in Europe for some hundreds of years.  Unlike the Transverse Mercator, this projection is not truly conformal and, therefore, the Transverse Mercator is now preferred over the Cassini.

This projection requires an Origin Latitude and a Central Meridian (i.e. origin longitude).  Unlike the Transverse Mercator, it does not use a scale reduction factor.


Modified Stereographic Projection

This projection is the result of serious map projection research by Miller, Laborde, Lee, Snyder, and others over a period of many years.  The projection is actually what is often referred to as a double projection.  The geographic coordinates are first projected on to a plane using a basic Stereographic formulation, and then further manipulated by sophisticated mathematics to achieve a specific objective, such as a minimum error map for a confined region.

Thus, the Stereographic portion of all this requires an Origin Latitude and Origin Longitude.  The mathemagical part requires a list of as many as 24 coefficients that are specified on the Parameter Pages of the Coordinate System Editor.


New Zealand National Grid

The New Zealand National Grid is another example of the implementation of the techniques described immediately above for the Modified Stereographic, but in this case the standard Mercator projection was used as the base projection.  In this case, however, the coefficients have been hard coded into the Coordinate Conversion System, leaving only the Origin Latitude and Origin Longitude as parameters.

It is highly unlikely that this projection would ever be used for any geography other than New Zealand, so why the Origin Latitude and Origin Longitude were not hard coded into the Coordinate Conversion System as well is a very good question.


Robinson Cylindrical Projection

The Robinson is another of several projections developed for the purpose of mapping the entire world.  Like most other projections developed for this purpose, it presumes an origin latitude of zero and requires an Origin Longitude parameter which specifies the center of any map produced with the resulting coordinate system.

Note that this projection is implemented only for the sphere, and is usually cartographically referenced directly to an ellipsoid.  If referenced to an ellipsoid (rather than a sphere), the Coordinate Conversion System will use the equatorial radius of the ellipsoid as the radius of the sphere.


Bonne Pseudoconical Projection

This projection requires an Origin Latitude and Origin Longitude. As usual, the origin longitude specifies the center of any map produced using the resulting coordinate system.

Interestingly, when the origin latitude is set to zero, the result is identical to the Sinusoidal Projection.  When the origin latitude is set to either pole, the result is also known as the Werner Projection.


Ekert Pseudocylindrical Projection, Number IV

The Ekert IV (there were six altogether) is another of several projections developed for the purpose of mapping the entire world.  Like most other projections developed for this purpose, it presumes an origin latitude of zero and requires an Origin Longitude parameter which specifies the center of any map produced with the resulting coordinate system.

Note that this projection is implemented only for the sphere, and is usually cartographically referenced directly to an ellipsoid. If referenced to an ellipsoid (rather than a sphere), the Coordinate Conversion System will use the equatorial radius of the ellipsoid as the radius of the sphere.


Ekert Pseudocylindrical Projection, Number VI

The Ekert VI (the last of the series) is another of several projections developed for the purpose of mapping the entire world.  Like most other projections developed for this purpose, it presumes an origin latitude of zero and requires an Origin Longitude parameter which specifies the center of any map produced with the resulting coordinate system.

Note that this projection is implemented only for the sphere, and is usually cartographically referenced directly to an ellipsoid. If referenced to an ellipsoid (rather than a sphere), the Coordinate Conversion System will use the equatorial radius of the ellipsoid as the radius of the sphere.


Mollweide Projection

The Mollweide, also known as the Homolographic, is another of several projections developed for the purpose of mapping the entire world, often in interrupted form.  Currently, the Coordinate Conversion system does not support the interrupted form of this projection, but such support is expected soon.

This projection presumes an origin latitude of the equator and requires that you specify the Origin Longitude which will become the center of any map based on the resulting coordinate system.

Note that this projection is implemented only for the sphere, and is usually cartographically referenced directly to an ellipsoid. If referenced to an ellipsoid (rather than a sphere), the Coordinate Conversion System will use the equatorial radius of the ellipsoid as the radius of the sphere.


Goode Homolosine Projection

The Goode Homolosine is another of several projections developed for the purpose of mapping the entire world, often in interrupted form.  It actually consists of using the Sinusoidal between latitudes of 40 South to 40 North (approximately) and the Mollweide (a.k.a. Homolographic) for the remainder of the earth.  Currently, the Coordinate Conversion System does not support the interrupted form of this projection, but such support is expected soon.

This projection presumes an origin latitude of the equator and requires that you specify the Origin Longitude which will become the center of any map based on the resulting coordinate system.

Note that this projection is implemented only for the sphere, and is usually cartographically referenced directly to an ellipsoid. If referenced to an ellipsoid (rather than a sphere), the Coordinate Conversion System will use the equatorial radius of the ellipsoid as the radius of the sphere.


Normal Aspect, Equal Area Cylindrical Projection

The Normal Aspect, i.e. equatorial aspect, of the Equal Area Cylindrical can be considered to be the equal area version of the conformal Mercator Projection.  Like the Mercator, it presumes an origin latitude of zero, requires an Origin Longitude, and uses a Standard Parallel to achieve the shrinkage affect.


Transverse Aspect, Equal Area Cylindrical Projection

The Transverse Aspect of the Equal Area Cylindrical can be considered to be the equal area version of the conformal Transverse Mercator Projection.  Like the Transverse Mercator, it requires an Origin Latitude, an Origin Longitude, and a Scale Reduction Factor.  Since this projection is relatively new to the Coordinate Conversion System, the Origin Longitude parameter is used rather than a Central Meridian parameter to define the center of the projection.


Bipolar Oblique Conformal Conic Projection

The Bipolar Oblique Conformal Conic is a projection invented for a single mapping project.  As such, one can legitimately question why the parameters were not simply hard coded into the Coordinate Conversion System.  In any case, it is unlikely that the projection would be used for any other purpose and our discussion will be quite vague.

This projection was developed specifically to map the Western Hemisphere, i.e. North and South America on a single map.  It essentially consists of two oblique conic projections joined along a specific line.  The parameters define the location of the apex of the two cones, and the angular distance from each apex to the standard parallels for each cone.

Note that this projection is implemented only for the sphere, and is usually cartographically referenced directly to an ellipsoid.  If referenced to an ellipsoid (rather than a sphere), the Coordinate Conversion System will use the equatorial radius of the ellipsoid as the radius of the sphere.


Swiss Oblique Cylindrical Projection

Surprise, surprise!!!  This projection is used in Switzerland.  It is, essentially, another formulation of the Oblique Mercator problem, where the central geodesic is defined such that it passes through the origin and achieves its maximum latitude at that point.

The projection requires an Origin Latitude and an Origin Longitude.  It does not use a scale reduction factor.


Krovak Oblique Conformal Conic, Czechoslovkia

This is an implementation of the obliaue conic concept as developed by Krovak for the mapping of Czechoslovokia.  This implementation uses the double projection technique where the ellipsoid is first projected onto a gaussian sphere,  the sphere is then treated as an oblique sphere, and the oblique sphere is then projected onto a cone using the Lambert Conformal Conic Single Parallel (with scale reduction) technique.

Users must specify most of the standard paratemeters.  The Origin Longitude and the Origin Latitude must be specified and determine the origin of the projection.  Note that the  Origin Latitude is the latitude at which the radius of the gaussian sphere is determined.  A Scale Reduction may be specified, the value of 0.9999 was used by Krovak to map Czechoslovokia.

Users must also specify the Oblique Pole Longitude and the Oblique Pole Latitude.  These values indicate the location on the spherical earth of the north pole of the oblique sphere of the projection.  Finally, the Oblique COne STandard Parallel must be given, it being the latitude on the oblique sphere at which the projection cone is tangent to the sphere.


Krovak Oblique Conformal Conic, Czechoslovkia

This is a variation of the Krovak Oblique Conformal Conic projection as developed by Krovak for mapping Czechoslovokia.  This variation adds the 1995 affine adjustment to the calculcated cartesian coordinates consistent.  The parameters for the adjustment are hard coded into the system and cannot be changed by the user.


Non-georeferenced (aka non-earth) coordinate system

This is the second of the pseudo projections.  Not a projection in the real sense, it is another trick on the system so that non-georeferenced coordinates can be processed by the system, with minial changes made.  In particular, this trick enables the system to process coordinates which are not georeferenced but yet apply a units conversion to them, as well as a translation.

This coordinate system is special in yet another way in that since the coordinates are not georeferenced, you cannot convert these coordinates to anyother georeferenced system.  You can only convert these coordinates to another coordinate system based on this pseudoprojection.  In this "pseudo conversion", the process operates as follows:

1.	The false easting and false northing specified in the source coordinate system definition are subtracted from the the source coordinates.

2.	The appropriate unit scaling, as determined by the source coordinate system units and the target coordinate system units, is applied.

3.	The false easting and false northing specified in the target coodinate system definition are added to the values to arrive at the converted result.

This projection only requires two prarameters in addition to the linear units specification.  These are the False Easting and False Northing.

Note that if this projection is used inappropriately the results will usually be very very small numbers.  This is the trick the coordinate conversion system uses to insure that this projection is not used inappropriately without the user being aware of it.


Oblique Cylindrical Projection (Generalized)

This projection is used in Hungary and elswhere (Romania?).  It is essentially a double projection implementation to to achieve the effect of an Oblique Cylindrical projection.  That is, the ellipsoid is first projected onto a gaussian sphere.  The pole of the gaussian sphere is then adjusted, and the oblique spherical result projected on to cylinder using the traditional Mercator algorithm.

This is the general case of the Oblique Cylindrical used in Switzerland, this variation accepting two additional parameters.  Use the traditional Origin Longitude and Origin Latitude parameters to define the projection origin in the traditional manner.  These parameters also defin the degree of obliqueness as the oblique sphere is such that its equator passes through the projection origin, and its prime meridian is made coincident with the Origin Longitude.   Use the Normal Parallel parameter to specify the latitude of the ellipsoid at which is the radius of the gaussian sphere is determined.  A Scale Reduction may also be specified, and it is implemented in the traditional Mercator manner.  That is, the Scale Reduction parameter could also be called the Scale of the Central Meridian.


Danish System 34, UTM + polynomials (pre-1999)

As its name implies, this projection is specialized for use in Denmark. This projection is yet another variation of the Transverse Mercator projection.  Essentially, the projection consists of a base UTM (zones 32 & 33) calculation, and then application of a high order (11th degree) polynomial to adjust the resulting UTM coordinates to produce the final result.  The coefficients of the polynomials are hard coded.

This projection accepts a single parameter which is the zone.  While the Danes would probably prefer to indicate the zone by name, or at worst a letter, parameters to projections supported by the coordinate conversion system must be numeric.  Therefore, the single parameter, Danish 34/45 Region, must be either 1, 2, or 3 as indicated in the following table.

Zone Name	Zone Letter	Zone Number
Jylland	J	1
Sjælland	S	2
Bornholm	B	3
Note, that the polynomials coded for this projection use values used prior to the 1999 recalculation of the polynomials.  Yet another variation of this projection has been implemented which uses the newer, 1999 polynomials values.


Danish System 34, UTM + polynomials (1999 vintage)

As its name implies, this projection is specialized for use in Denmark. This projection is yet another variation of the Transverse Mercator projection.  Essentially, the projection consists of a base UTM (zones 32 & 33) calculation, and then application of a high order (13th degree) polynomial to adjust the resulting UTM coordinates to produce the final result.  The coefficients of the polynomials are hard coded.

This projection accepts a single parameter which is the zone.  While the Danes would probably prefer to indicate the zone by name, or at worst a letter, parameters to projections supported by the coordinate conversion system must be numeric.  Therefore, the single parameter, Danish 34/45 Region, must be either 1, 2, or 3 as indicated in the following table.

Zone Name	Zone Letter	Zone Number
Jylland	J	1
Sjælland	S	2
Bornholm	B	3
Note, that the polynomials coded for this projection use values determined in 1999 and supercede the 11th order polynomials used previously.  Yet another variation of this projection has been implemented which uses the older, pre-1999 polynomials values.


Ordnance Survey National Grid Transformation of 1997

Yet another variation of the Transverse Mercator projection, this variation adds the X and Y data shifts as determined by the OSTN97 (Ordnance Survey National Grid Transformation of 1997 [sic])  calculation and grid file.  Thus, this projection should only be used to convert ETRF89 geographic coordinates to OSGB coordinates.  The intent (I believe) is to continoue using OSGB36 coordinates, even though surveyors will be capturing/calculating EFTR89 geographic coordinates.

Note, that this variation converts from ETRF89 geographic coordinates to OSGB36 coordinates using the OSTN97 algorithm and data file.  Another variation exists which performs the same function, but uses the OSTN02 algorithm and data file.  The two produce similar, but not prcisely the same, results

This projection takes no parameters as the standard OSGB parameters are hard coded into the projection.  Note that any attempted use of this projection while the OSTN97.TXT file is not installed in the main coordinate conversion system data directory will fail.  The file must have this exact name (and case on case sensitive operating systems).  The file must exists in the same directory as the basic dictionary files.


Ordnance Survey National Grid Transformation of 2002

Yet another variation of the Transverse Mercator projection, this variation adds the X and Y data shifts as determined by the OSTN02 (Ordnance Survey National Grid Transformation of 2002 [sic])  calculation and grid file.  Thus, this projection should only be used to convert ETRF89 geographic coordinates to OSGB coordinates.  The intent (I believe) is to continoue using OSGB36 coordinates, even though surveyors will be capturing/calculating EFTR89 geographic coordinates.

Note, that this variation converts from ETRF89 geographic coordinates to OSGB36 coordinates using the OSTN02 algorithm and data file.  Another variation exists which performs the same function, but uses the OSTN97 algorithm and data file.  The two produce similar, but not prcisely the same, results

This projection takes no parameters as the standard OSGB parameters are hard coded into the projection.  Note that any attempted use of this projection while the OSTN02.TXT file is not installed in the main coordinate conversion system data directory will fail.  The file must have this exact name (and case on case sensitive operating systems).  The file must exists in the same directory as the basic dictionary files.


Geodetic Reference

In this section the nature and use of the geodetic transformation techniques supported by the coordinate conversion system are reviewed.  This information is rather technical in nature and is intended for those with some experience in using geodetic transformation techniques.

In all cases, user defined geodetic transformations are a mathematical processes by which geographic coordinates can be converted from the datum being defined to WGS84.  While there are exceptions to the rule, most all datum definitions are of this form.  The algorithms implemented in this product require that datum definitions be given in this form.  While it is know that there are a few particular datums where "passing through WGS84" is undesirable, an alternative to this general concept is not available at this time.

The following sections describe the geodetic transformation methods made available by the coordinate conversion system.

In This Section  Bursa/Wolf (aka Helmert) Transformation  Seven Parameter Transformation  Three Parameter Transformation  Molodensky  Multiple Regression   Is NAD83, No Shift Required  Is GDA94, No Shift Required  Is NZGD2K, No Shift Required  Is WGS84, No Shift Required  WGS72 to WGS84  Six Parameter Transformation  Four Parameter Transformation  NAD27 to NAD83 via NADCON  HARN to NAD83 via NADCON  NAD83 to CSRS via Grid File  AGD66 to GDA94  AGD84 to GDA94  NZGD49 to NZGD2K via Grid File  ED50 to ETRF89 via Grid Files  ATS77 to CSRS via Grid File  Tokyo to JGD2k via Grid Files  NTF to RGF93 via Grid File  NAD27 To CSRS via Grid File	See Also  Coordinate Conversion System   Overview  Coordinate System Referencing  Coordinate System Key Name Conventions  Dictionary Entry Protection  Interactive Dialog Support  Interactive Dialogs  Supported Projections  Geodetic Data Catalog


Bursa/Wolf (aka Helmert) Transformation

This transformation is actually an approximation of The Seven Parameter Transformation that has been traditionally used for geodetic transformation.  This approximation has been used for many years, and is the transformation most often documented in less rigorous geodetic documentation.  The approximation is very good as long as the three rotation angles are rather small, on the order of two or three seconds of arc.  When rotation angles exceed these values, the results begin to strain normally expected levels of accuracy.

The approximation referred to above is arrived at by making three assumptions: 1) the sine of a small angle is equal to the angle (in radians) itself; 2) the multiplication of two sine terms is zero; and the cosine of a small angle is one.  As you can see, this approximation is valid only for small angles.

In all other aspects, this transformation is the same as the Seven Parameter transformation.  In processing new data projects, use the Seven Parameter transformation in lieu of the Bursa/Wolf.  The Bursa/Wolf approximation is provided for purpose of providing the means to reproduce numbers/calculations that were originally accomplished using the approximation.

Please note that the terminology used here is specific to the coordinate conversion system in this product.  That is, the use of the term Bursa/Wolf to refer to the approximate Seven Parameter Transformation (rather than the rigorous Seven Parameter Transformation described above) is not generally accepted in the industry.  The author of this manual is not aware of generally accepted terminology to distinguish between these two similar, but different, transformations.

See Also

Geodetic Reference

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


Seven Parameter Transformation

This transformation is a rigorous implementation of the standard three dimensional transformation.  Obviously, it gets its name from the number of parameters that are required.  As indicated above, the provided parameters must indicate the transformation to convert local datum coordinates to WGS84 coordinates.  For many typical GIS applications, you can simply change the sign of each of the seven parameters to effect an inverse.  However, this technique is not exact.  For precise results, a rigorous inversion is necessary in order to determine the appropriate parameters.

Essentially, this transformation proceeds in three phases.  First, the geographic coordinates are converted to three dimensional Cartesian, geocentric coordinates using the ellipsoid of the original datum.  Second, the three dimensional transformation defined by the seven parameters is applied producing a modified set of geocentric Cartesian coordinates.  Third, the resulting geocentric coordinates are converted back to geographic form using the WGS84 ellipsoid.

The seven parameters are:

Delta X: the amount the intermediary geocentric X coordinate is translated.  This value must be given in meters and the direction of the translation is given by the sign of the value.

Delta Y: the amount the intermediary geocentric Y coordinate is translated.  This value must be given in meters and the direction of the translation is given by the sign of the value.

Delta Z: the amount the intermediary geocentric Z coordinate is translated.  This value must be given in meters and the direction of the translation is given by the sign of the value.

X Rotation: the amount of rotation about the X axis which is applied to the intermediary geocentric coordinates.  This value is given in seconds of arc, and the direction of the rotation is indicated by the sign of the value.

Y Rotation: the amount of rotation about the Y axis which is applied to the intermediary geocentric coordinates.  This value is given in seconds of arc, and the direction of the rotation is indicated by the sign of the value.

Z Rotation: the amount of rotation about the Z axis which is applied to the intermediary geocentric coordinates.  This value is given in seconds of arc, and the direction of the rotation is indicated by the sign of the value.

Scale: a scale factor that is applied to the intermediary geocentric coordinates.  The value is given as a value in parts per million and is the difference of the actual scale factor and unity.  For example, a value for the scale parameter of –2.5 produces an actual scale factor of 0.9999985.  That is, the actual scale factor used is arrived at by multiplying the parameter value by 1.0x10-06 and adding the result (algebraically) to 1.0.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


Three Parameter Transformation

Naturally, this transformation requires three parameters.  Effectively, use of this transformation will produce the same results as the Seven Parameter Transformation with all three rotation parameters and the scale parameter set to zero.

As with the Seven Parameter Transformation, this transformation proceeds in three phases.  First, the geographic coordinates are converted to three dimensional Cartesian, geocentric coordinates using the ellipsoid of the original datum.  Second, the three translation parameters, Delta X, Delta Y, and Delta Z, are used to translate the geocentric coordinates.  Third, the resulting geocentric coordinates are converted back to geographic form using the WGS84 ellipsoid.

As in all other cases for the translation parameters, the three parameters must be given in units of meters.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


Molodensky

This transformation is the DMA (US Defense Mapping Agency, aka NIMA) implementation of the Molodensky transformation.  (The formulas used were extracted from Defense Mapping Agency Technical Report 8350.2-B, 1 December 1987.)  Effectively, it is variation of the Three Parameter Transformation that produces very similar results and can be calculated without iteration.  Most importantly, the parameter use for this transformation is the same as the Three Parameter Transformation described immediately above.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


Multiple Regression

This transformation is based on the series of Multiple Regression developments published by the US Defense Mapping Agency (now known as NIMA) in Technical Report 8350.2-B, December 1987.  Essentially, these are formulas developed from applying linear regression techniques to a varying number of points where the local coordinates and WGS84 coordinates are rather precisely known.

These regression formulas are based on normalized input coordinates.  It is assumed that the normalized coordinates define the useful range of the datum transformation.  In theory, therefore, a geographic coordinate which produces a normalized coordinate greater than 1.0 or less than –1.0 would normally be considered to be outside the useful range of the transformation.  In this implementation of the regression technique, a geographic coordinate is considered to be outside the useful range of a transformation if the absolute value of either normalized coordinate exceeds 1.4.

In the event that a coordinate is given which is outside of the useful range of the multiple regression formula as described above, a fallback technique is used to calculate a datum shift.  In this case, the fallback technique is the Molodensky, the Six Parameter Transformation, or the Seven Parameter Transformation depending upon how many parameters have been defined in the base definition.  That is, when defining the datum definition, temporarily set the technique specification to Seven Parameter and set the desired fallback parameters.  Then, the technique can be set back to the Multiple Regression selection and the parameters values will be preserved.

Currently, the parameters to such a transformation consist of a preprocessed .MRT file.  These files contain all of the coefficients of the multiple regression formula in a compact form.  This form also facilitates the actual testing of each parameter file individually as the DMA provided test case is included in the file.  Currently, no provisions are made for users to implement their own multiple regression parameter files.  This may change in the future.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


Is NAD83, No Shift Required

For practical GIS applications, there is no difference between NAD83 and WGS84.  Both are very precise measurements of the same thing, and what differences there are between the two are largely due to how the statistical noise was handled.  Also, there are no published techniques or generally accepted means of converting from NAD83 to WGS84.  The end result of all this is that this transformation currently does nothing.  If, in the future, a generally accepted means of converting from NAD83 to WGS84 becomes available, the transformation will be modified to implement that technique.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


Is GDA94, No Shift Required

At the current time, the differences between GDA94 and WGS84 are small.  Further, a generally accepted means of converting between GDA94 and WGS84 is unknown to the authors of the coordinate conversion system.  This technique currently does nothing.  It acts only as a placeholder that will eventually perform the conversion from GDA94 to WGS84 when an appropriate technique for performing this conversion is located.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


Is NZGD2K, No Shift Required

At the current time, the differences between NZGD2K and WGS84 are small.  Further, a generally accepted means of converting between NZGD2K and WGS84 is unknown to the authors of the coordinate conversion system.  This technique currently does nothing.  It acts only as a placeholder that will eventually perform the conversion from NZGD2K to WGS84 when an appropriate technique for performing this conversion is located.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


Is WGS84, No Shift Required

This is another transformation that essentially does nothing, silently (i.e. never an error), and very quickly.  Obviously, the WGS84 datum definition refers to this transformation technique.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


WGS72 to WGS84

This transformation implements the formulae published by the U. S. Defense Mapping Agency in Technical Report 8350.2-B, December 1987 for transforming WGS72 based geographic coordinates to WGS84 based coordinates.  The transformation is hard coded and does not require any parameters.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


Six Parameter Transformation

As you might expect, this transformation is the Seven Parameter Transformation without one of the parameters.  The omitted parameter is the Scale parameter.  You could achieve the same results by using the Seven Parameter Transformation, setting the scale parameter to zero, and setting the remaining six parameters as appropriate.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


Four Parameter Transformation

As you might expect, this transformation is the Seven Parameter Transformation without three of the parameters.  The omitted parameters are the Rotation parameters.  You could achieve the same results by using the Seven Parameter Transformation, setting the three rotation parameters to zero, and setting the remaining four parameters as appropriate.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


NAD27 to NAD83 via NADCON

This transformation represents the integration of the algorithms originally published by the National Geodetic Survey of the US and Geomatics Canada in the forms commonly known as the NADCON program and the National Transformation (Versions 1 and 2).  That is, this transformation is the means by which one would convert from the North American Datum of 1927 (NAD27) to the North American Datum of 1983 (NAD83).

All of the techniques encapsulated in this transformation rely on access to data files that define the amount of the shift from NAD27 to NAD83 in a grid form.  The coordinate conversion system uses algorithms identical to those used by the respective government published programs to interrogate the data files and determine the shift for any given coordinate.

Note that the coordinate conversion system considers NAD27 and NAD83 to be a single entity.  That is, no distinction is made between Canadian NAD83 or US NAD83 with regard to this technique.  In typical operation, the coordinate conversion system will first determine which of the several data files that may be present actually contain coverage for the point to be converted.  In the case where one or more files provide coverage, the data file with the densest grid (assumed to be the most accurate) is selected for the calculation.  The Geodetic Catalog feature of the coordinate conversion system can be used to specify the data files which are to be considered for use, and a means to control which file takes precedence in the case where one or more of the data  files overlap.  Refer to Geodetic Data Catalogs for more information.

Note that this transformation implies a conversion from NAD27 to NAD83.  The coordinate conversion system will automatically complete the conversion to WGS84 by also causing the
"Is NAD83, No Shift Required" technique after the NADCON/National Transformation technique is done.

The shift data is stored in a single data file for the Canadian National Transformation (either version) and neither of these data files is in the public domain.  These data files cannot be provided as part of the normal distribution of this product and must be obtained from Geomatics Canada, and a license fee is often required.  In the case of the US NADCON data files, two files are required fro each region covered.  One file contains the latitude shift and the second contains the longitude shift.  These files are in the public domain and are usually included in the distribution of this product.  Should updates become available, you can use the data files in the exact form as they are published buy the National Geodetic Survey.

Since the coverage of the data files is limited, the coordinate conversion system uses a fallback technique to calculate datum shifts for coordinates that are not covered by the data files.  Users indicate the desired fallback technique through the use of the Geodetic Data Catalog file.  The Geodetic Data Catalog file named Nad27ToNad83.gdc contains the list of data files to be used for this transformation.  Refer to Geodetic Data Catalogs for more information.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


HARN to NAD83 via NADCON

HARN is an acronym for High Accuracy Reference Network.  This datum has also been known as HPGN.  HPGN is an acronym for High Precision GPS Network.  Both of these terms refer to the same thing.  This 'thing' has also been referred to as NAD83/91.  This 'thing' is a rework of NAD83 with the aid of GPS technology, as GPS was not functional in 1983.  This technique selection implies the use of the algorithms and data files of the U. S. National Geodetic Survey's NADCON program to effect the shift between NAD83 and HARN.

As with the NADCON technique described above, this transformation relies on the existence of data files that define the shift at various geographic points in a grid format.  As is the case with the NAD27/NAD83 NADCON data files, these data files come in pairs and are in the public domain.  A Geodetic Data Catalog file is used to inform the coordinate conversion system of which files are to be considered for use, and the location of these data files.

All of the data files used in this transformation adhere to a specific naming convention.  (That is, as published by the National Geodetic Survey, the adhere to this convention.  The coordinate conversion system cares not about the names or locations of the files; as long as the files have the proper .LAS and .LOS extensions and the names and locations are properly recorded in the Geodetic Data Catalog file.)  These files all overlap their neighbors be a substantial amount.  When originally introduced, the HARN data files would always produce identical results in the regions of overlap.  However, as the system was completed, this is no longer true.  Therefore, different results for the same point can be obtained depending upon which specific data files are used.

Therefore, users should pay significant attention to the set up of the Geodetic Data Catalog for specific data conversions.  For example, if the geography one is working is primarily in Ohio, then the Ohio HPGN file should be listed first in the catalog file.  This will cause that data file to take precedence over all others in the case of overlap.

Users can use the Geodetic Data Catalog file named HarnToNad83.gdc to specify a fallback definition to be used when coordinate data not covered by the data files is processed.  Refer to Geodetic Data Catalogs for more information.  Since the shift produced by this transformation is relatively small (e.g. one or two feet), the fallback feature is rarely used.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


NAD83 to CSRS via Grid File

CSRS is an acronym for Canadian Spatial Reference System; the Canadian equivalent to the US's HARN.  That is, a very accurate rework of NAD83 using GPS technology.  Like the US HARN, the shifts are in the range of one to two feet (i.e. 40 centimeters).

This conversion technique is implemented by a series of datum shift grid files of the Canadian National Transformation format.  Unlike other implementations by the Canadians, however, there are multiple overlapping files involved.  So, surprise, surprise; this situation is managed by the Geodetic Data Catalog feature.  In this case, the name of the data catalog file is Nad83ToCsrs.gdc.  Refer to Geodetic Data Catalogs for more information.

You can use the fallback feature of the Geodetic Data Catalog to specify how data points outside the coverage of existing data files are to be handled.  Since the shift produced by this transformation is relatively small (e.g. one or two feet), this fallback feature is rarely used in this case.

Again, the data files are being generated on a province by province basis.  The individual files may, or may not, be in the public domain.  You may need to acquire the appropriate data files yourself in order to use the transformation.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


AGD66 to GDA94

The Australians have adopted the techniques developed by Geomatics Canada for its National Transformation (Version 2) to define a precise means of converting from the Australian Geodetic Datum of 1966 to the newer Geocentric Datum of Australia 1994.  The data files involved are listed in the Geodetic Data Catalog file named Agd66ToGda94.gdc.  Refer to Geodetic Data Catalogs for more information.  Again, the areas of overlap deserve special attention.

Datum shift data files for this transformation are being developed on a state by state basis.  Therefore, there are (and for some time will remain) more than one data file for this transformation.  Again, the coordinate conversion system considers AGD66 to be a single entity even through there are several different data files which overlap.  Users are encouraged to use the features of the Geodetic Data Catalog concept to insure that the desired data file is used in the regions of overlap.

Also note that the Geodetic Data Catalog feature can also be used to establish a fallback to be used to convert a data point which is not covered by any of the data files.

Use of this transformation implies the use of the GDA94 to WGS84 transformation technique (as described above) automatically to produce WGS84 coordinates.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


AGD84 to GDA94

Several of the states in Australia have been using the Australian Geodetic Datum of 1984 for some time now.  This transformation technique implements the conversion of AGD84 to GDA 1994.  Operationally, this technique is identical to the AGD66 < - > GDA94 with the exception that a different Geodetic Data Catalog is used (Agd84ToGda94.gdc in this case) and the actual data files that are used are different.  Refer to the description of the AGD66 to GDA94 transformation from more details.

Use of this transformation implies the use of the GDA94 to WGS84 transformation technique (as described above) automatically to produce WGS84 coordinates.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


NZGD49 to NZGD2K via Grid File

New Zealand has also adopted the Canadian technique to define the shift from the New Zealand Geodetic Datum of 1949 (NZGD49) to New Zealand Geocentric Datum of 2000 (NZGD2K).  This implementation is somewhat simpler in that only a single data file is used.  However, in order to use the same program code that was implemented for Canadian and Australian geography, a Geodetic Data Catalog file is used even though only a single file is listed.  The Geodetic Data Catalog file used is named Nzdg49ToNzgd2K.gdc.  Refer to Geodetic Data Catalogs for more information.

Use of this transformation implies the use of the NZGD2K to WGS84 transformation technique (as described above) automatically to produce WGS84 coordinates.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


ED50 to ETRF89 via Grid Files

ED50 refers to the European Datum of 1950.  ETRF89 refers to Eurpean Terrestrial Reference Frame of 1989.  ED50 has been used in (surprise) Europe for many years.  ERRF89 is essentially equivalent to WGS84 with slight modifications to account for the tectonic movement of the continental plate(s).  Thus, WGS84 and ETRF89 are, for most folks, the same thing.  The actual differences are measured in centimeters, and only specialized applications use data with that degree of accuracy.

As of this writing, we know of only one grid data file in this class, and it is not official.  Also, we have been unable to locate any official reference to this type of datum transformation.  All we have is a legitimate data file in the Canadian National Transformation, Version 2, (i.e. .gsb) format.  So, this feature may not be of any value at the current time.  Should any grid data files for this transformation ever become official, and they are published in the Canadian National Transformation, Version 2, format, you simply need to add a reference to the data file to the Geodetic Data Catalog file named Ed50ToEtrf89.gdc and you're in business.  (Note, if such a file does not exist on your system, simply create one.  Geodetic Data Catalogs are simply text files and can be created with Notepad.  The actual grid files can reside in any folder on your system.  The Geodetic Data Catalog must reside in the principal data directory which contains the dictionary files.)

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


ATS77 to CSRS via Grid File

The Average Terrestrial System of 1977 has been used in the maritime provinces of Canada since (surprise) 1977.  This transformation uses data files in the Canadian National Transformation (version 2) format to determine the shift required to properly transform ATS77 based geographic coordinates to CSRS based coordinates.  Note that:

•	the result of using the grid shift data alone is CSRS (Canadian Spatial Reference System) and not WGS84.  The Coordinate Conversion System then proceeds to convert the CSRS coordinates to WGS84.

•	each of the different provinces involved have produced a data file covering the geography of their respective provinces,

•	the individual data files are not in the public domain and must be obtained by users directly from the provincial governments involved, and

•	a Geodetic Data Catalog file is used to manage the multiple file situation.

The Geodetic Data Catalog file involved in this transformation is Ats77ToCsrs.gdc.  Refer to Geodetic Data Catalogs for more information.  The fallback feature of the Geodetic Data Catalog is fully supported by this technique.

Refer to Maritime Provinces, Special Case for more information related to ATS77 and the Maritime Provinces.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


Tokyo to JGD2k via Grid Files

Use this technique to select a conversion from the Tokyo Datum to the Japanese Geodetic Datum of 2000 using one or more of the grid shift definition data files created for this conversion.  For all practical purposes, the Japanese Geodetic Datum of 2000 is considered to be the same as WGS84.  While there are differences, the magnitude of these differences are not likely to adversely affect the typical GIS application.  Note that:

•	the grid shift files are basically text files in a specific data format;

•	the data files we were provided with for testing had the .par extension;

•	from examining the data format, we believe it is likely that a single, rather large, file could be constructed to cover the entire nation of Japan;

•	and that smaller files could also be constructed to cover smaller regions;

•	since our Japanese is a but rusty, we have not been able to determine exactly how or where official data files cane be obtained; and

•	a Geodetic Data Catalog file is used to manage the multiple file situation.

•	the Geodetic Data Data file is named TokyoToJgd2k.gdc.

The Geodetic Data Catalog file involved in this transformation is TokyoToJgd2k.gdc.  Refer to Geodetic Data Catalogs for more information.  The fallback feature of the Geodetic Data Catalog is fully supported by this technique.

Data in the original textual format is difficult to deal with in an efficient manner, and this data does not change very often, if ever.  Therefore, when first asked to use data in any of these files, the Coordinate Conversion System will make a binary copy which it will use for future calculations.  The first time you use the Coordinate Conversion System to convert data using this technique, this copy process will cause a slight pause in the calculation process.  Once the binary copies are made, they are retained and will be used until such time as the Coordinate Conversion System detects that the original data files have been changed or updated.  Thus, this pause should only occur the first time this technique is used.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

NTF to RGF93 via Grid File

NAD27 To CSRS via Grid File


NTF to RGF93 via Grid File

NTF is an acronym for Nouvelle Triangulation de la France, and RGF93 is an acronym for Réseau Gédésique Français en 1993.  These are French reference systems, and RGF93 closely resembles WGS84 and is considered by most to be equivalent to WGS84.

This transformation uses data files in a format specific to this transformation (although it is likely that this format will be chosen by others for future datum shift development).  We only know of one official data file and this file covers the entire country of France.  Note that:

•	since there is only one data file, a Geodetic Data Catalog is not used for this transformation;

•	since there is no Geodetic Data Catalog, the single data file must be named gr3df97a.txt;

•	since there is no Geodetic Data Catalog, the single data file must be located in the primary data folder searched by the Coordinate Conversion System;

•	the data file can be obtained from the IGN web site (see link below).

Data in the original textual format is difficult to deal with in an efficient manner, and this data does not change very often, if ever.  Therefore, when first asked to use the data file, the Coordinate Conversion System will make a binary copy which it will use for future calculations.  The first time you use the Coordinate Conversion System to convert data using this technique, this copy process will cause a slight pause in the calculation process.  Once the binary copies are made, they are retained and will be used until such time as the Coordinate Conversion System detects that the original data files have been changed or updated.  Thus, this pause should only occur the first time this technique is used.

Accessing datum shift data files without using the Geodetic Data Catalog concept is inconsistent with the rest of the system.  At least several clients have express that opinion.  Therefore, it is likely that a future release of the Coordinate Conversion System will use the Geodetic Data Catalog feature to enable the single data file to have a name other than that specified here, and enable the file to reside in a folder other than the primary data folder.

Link to the data file:

http://www.ign.fr/telechargement/Pi/SERVICES/gr3df97a.txt

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NAD27 To CSRS via Grid File


NAD27 To CSRS via Grid File

Several of the Canadian provinces have published grid shift files in the National Transformation, Version Two, format (i.e. .gsb) which convert directly from NAD27 to CSRS (Canadian Spatial Reference System), thus bypassing the intermediary NAD83.  Select this technique to implement a conversion using such a file.

The actual files used, the locations on your system, and the order in which they are searched, are specified in a Geodetic Data Catalog file named Nad27ToCsrs.gdc.  This conversion is only active when converting directly between these two datums.  Since CSRS implies Canada, use of this technique for geography outside of Canada is inappropriate.

There are some special considerations with regard to this conversion when applied to geography in the Maritime Provinces of Canada.  Refer to Maritime Provinces, Special Case for more detailed information.

See Also

Geodetic Reference

Bursa/Wolf (aka Helmert) Transformation

Seven Parameter Transformation

Three Parameter Transformation

Molodensky

Multiple Regression

Is NAD83, No Shift Required

Is GDA94, No Shift Required

Is NZGD2K, No Shift Required

Is WGS84, No Shift Required

WGS72 to WGS84

Six Parameter Transformation

Four Parameter Transformation

NAD27 to NAD83 via NADCON

HARN to NAD83 via NADCON

NAD83 to CSRS via Grid File

AGD66 to GDA94

AGD84 to GDA94

NZGD49 to NZGD2K via Grid File

ED50 to ETRF89 via Grid Files

ATS77 to CSRS via Grid File

Tokyo to JGD2k via Grid Files

NTF to RGF93 via Grid File


Geodetic Data Catalog

Datum conversions that rely on the existence of one or more grid data files use the concept of a Geodetic Data Catalog to provide the name and location of the actual grid data files which are to be used.  An ordinary text file with the .gdc extension is used for this purpose.  These files are also referred to as .gdc files.

Since these files are simple text files, they can be created and/or modified using any text editor, such as Notepad.  You can use a word processor if you are careful to save the file as an ordinary text file.  Each line of text in the file provides the Coordinate Conversion System with the name of the file, the location of the file, and some information as to how the file is to be used.  Up to four data elements can be specified.  Only the first element, the file name and location is a required element.

In most cases, the Geodetic Data Catalogs provided by your software's distribution are fine for general use.  However, there are definite cases where you will want to tweak these files for your own unique situation.  These situations are the several cases where there are several data files involved, and the coverage of these files overlap.

For example, in the case of NAD27 to NAD83 (the letters NAD refer to North American Datum) conversion, there are several files.  The U. S. files by themselves do not present a problem as they do not overlap.  However, when we add the Canadian National Transformation file to the catalog, we have a lot of geography where the data files overlap.  If the use of either data file in the case of overlap produced the same results, we would not have a problem.  However, this is rarely the case.  Therefore, users often wish to specify which file gets used in the case of overlap.  This is one of the functions of the Geodetic Data Catalog.

For each point which is to be converted, the Coordinate Conversion System will look at all the files listed in the appropriate Geodetic Data Catalog.  In selecting a file to use, the Coordinate Conversion System will first determine which of the files listed actually cover the geographic location of the point being converted.  Of the files providing coverage,the Coordinate Conversion System will then select the files with the densest grid; that is the grid shift data file in which the grid cells are the smallest.  (It is presumed that the smaller the grid cell, the more accurate the data in the file.)  In the case where two or more files meet these initial criteria, the Coordinate Conversion System will choose the first file listed in the Geodetic Data Catalog.

While the Coordinate Conversion System can determine the grid density of most all datum shift grid files, users can use the Geodetic Data Catalog to override the actual grid density value to control the datum shift grid file selection process.  Further, users can alter the order in which the datum shift grid files are listed to further control the datum shift grid file selection process.  The following sections describe how one can use the interactive Geodetic Data Catalog Editor, or simply use a text editor on the .gdc file, to accomplish these tasks.

Geodetic Data Catalogs can also be used to improve the performance of the Coordinate Conversion System.  For example, the HARN data catalog file must contain as many as 22 file references.  If your data conversions are limited to a single state, the unused files may be removed and performance of HARN conversions improved somewhat.

In This Section  Catalog Contents  Fallback Datum  Maritime Provinces Special Case	See Also  Coordinate Conversion System  Overview  Coordinate System Referencing  Coordinate System Key Name Conventions  Dictionary Entry Protection  Interactive Dialog Support  Interactive Dialogs  Supported Projections  Geodetic Reference


Catalog Contents

The four data elements that appear on each line of text are as follows, the elements are separated from each other by commas.

File Name and Location  The first element must be the name and location of the file.  A fully qualified path name may be provided, the file extension must always be given. Path names do not represent a complete path name are considered to be relative to the folder (directory) in which the catalog file itself resides.

For NADCON file references, you need only reference one of the .LAS/.LOS files.  The Coordinate Conversion System will automatically load the companion file.

Buffer Size  Access to grid data files is buffered.  That is a large portion of the file is usually read into memory to prevent constant accessing of the hard disk drive during conversions.  You can use this element to override the default buffer size for a file.  The default buffer size is dynamically chosen for specific files, but does not exceed 150KB per file or the actual size of the file, which ever is smaller.  Buffer size may be specified in decimal or hexadecimal format.

Flags  Currently, the Flags element is unused.  In future revisions of the Coordinate Conversion System, the flags element may be used to provide users with additional control as to how these grid files are accessed.  Flags may be specified in decimal or hexadecimal format.

Grid Density  All grid files have a grid density specification in the file or associated with them by default.   Density is specified as the width/height of the grid cells in minutes of latitude and longitude. The Coordinate Conversion System uses this density to choose between files in situations where the files overlap each other.  The presumption here is that the file with the smallest grid cell (highest density) will produce the best results.  You can use this element to override the internal density setting and cause a specific file to be used in all overlap situations.  Grid density is specified as a single number.

See Also

Geodetic Data Catalog

Fallback Datum

Maritime Provinces Special Case


Fallback Datum

Fallback refers to the calculations to be performed when the primary calculation fails.  For example, when converting from NAD27 to NAD83, what should the system do when presented with a point that is not covered by any of the datum shift data files.  In these cases, you can instruct the system to invoke the fallback technique.  The fallback technique is a reference to another datum definition, but one based on a technique that does not usually have limitations (such as the Seven Parameter transformation).

A fallback definition consists of the name of a datum definition that exists in the Datum Dictionary.  Additionally, the fallback datum definition must not be based on a transformation technique that relies on data files.  That is, the fallback datum definition must be based on either the: Molodensky, Multiple Regression, Bursa/Wolf, Seven Parameter, Six Parameter, Four Parameter, or Three Parameter techniques.  These techniques are not subject to failure based on regional concerns.

To include a fallback specification in the catalog file, simply add a line of text that contains the keyword "Fallback" (case insensitive) and the datum name as indicated in the following example:

Fallback=NAD27-48

In this case, the name of the fallback datum is "NAD27-48".  This entry should appear on its own line in the data catalog file.

The astute user may have realized that the Multiple Regression technique can indeed fail on a regional basis.  That is, fail due to being asked to convert a coordinate outside the useful range of the conversion technique.  However, as described below, the Multiple Regression technique has its own fallback capability built into it.  Thus, datums based on the Multiple Regression technique which are to be used as a fallback should always have their own fallback feature invoked.

See Also

Geodetic Data Catalog

Catalog Contents

Maritime Provinces Special Case


Maritime Provinces Special Case

In the Maritime Provinces of Canada, there are four datums in current use.  Thus, providing conversions from any one to any of the others requires 12 different transformation possibilities.  As of this release, the Coordinate Conversion System supports eight of the twelve in a manner sanctioned by the individual governmental agencies in these provinces.  The remaining four transformations are not sanctioned by any of these governmental agencies, but are supported by the Coordinate Conversion System.  There can be no guarantee whatsoever that conversion involving the unsanctioned, but supported, conversions would be considered correct by anyone.

Unsanctioned Transformations

The unsanctioned (but supported) transformations are NAD83 · CSRS and ATS77 · NAD83.  This is largely because the Maritime provinces never officially adopted NAD83.  Conversions between NAD27 and NAD83 in the provinces are sanctioned, but only because the national government developed and sanctioned the transformation as part of the original National Transformation project.  To this date, conversions between NAD83 and CSRS are being accomplished by the individual provinces.  Since the Maritime Provinces never adopted NAD83, they will not be (so we are told) be producing a shift file for NAD83 · CSRS conversions.

The Coordinate Conversion System achieves conversion in these cases by assuming that CSRS and NAD83 are the same.  This assumption is good enough for many uses as the difference is known to never to exceed 40 centimeters.  However, users should be aware that in the Maritime Provinces, conversions to/from NAD83 for any datum other than NAD27 will be suspect.

NAD27 · ATS77

There exists rather unique transformation algorithm and associated data files for this conversion.  The algorithm (actually program) name is TRANSFORM and there are six data files; one file for each province and in each direction.  As of this writing, we are sure that we are permitted to distribute these data files.  You will need to obtain them from the provincial government(s).  Once these data files are made available, register their locations in the Nad27ToAts77.gdc Geodetic Data Catalog file and sanctioned results can be obtained.

The files which you will want to obtain have file names such as TRNB2777.dat or TRNB7727.dat.  The second two letters in the file name indicate the province to which the file applies.  The sequence of the 27 and 77 in the file name indicate the direction of the conversion supported by the data file.  Note that these files are the very same files you receive when you license the NbGeoCal program from Services New Brunswick.  Simply point to them from inside the Geodetic Data Catalog file.

Take care to notice that there exists a large degree of overlap among these files, and in the region of overlap, the different files do not produce the same results.  Therefore, you will want to pay strict attention to the order of the data files in the Geodetic Data Catalog.  Another approach to this problem is to simply register only the files which apply to the province in which you are working.  This will ensure proper results.

ATS77 · CSRS

This datum shift is sanctioned and is accomplished in the normal manner by the Coordinate Conversion System.  That is, a Geodetic Data Catalog is used to reference one or more grid data files in the standard National Transformation, Version 2, (i.e. .gsb) file format.  The Maritime Provinces do sanction ATS77 and CSRS.  These shift files are available from the provincial governments, and are the same files distributed with the NbGeoCalc program.  For example, NS7783.gsb provides the shift for Nova Scotia.  Again, be careful of the overlap condition, and be sure to either order the files in the Geodetic Data Catalog file properly, or limit your reference to one file/province that is appropriate for your work.

NAD27 · NAD83

This conversion is handled in the normal manner.  In fact, if you have access/license to use the standard National Transformation, Version 2, data file, you need not do anything special.  Be aware, that while the NbGeoCalc distribution includes its own grid shift file for this conversion, the file itself is only a subset of the standard national file.  Typically, therefore, most users do not need to do anything to add this conversion to their application.

There is an overlap problem here as well, but the problem is overlapping with the US data files.  So again, take care to make sure that the references to the grid shift data files are in the order of preference you desire.  A sure way to force use of a specific grid file is to remove references to all the other files from the Geodetic Data Catalog.

 NAD27 · CSRS

This is particular conversion is sanctioned, and fully supported, but presents a problem which, admittedly, does not fit well with the original design of the Coordinate Conversion System.  The officially sanctioned means to perform this conversion is to convert the NAD27 coordinates to ATS77 using the algorithm discussed at length above, and then convert the resulting ATS77 coordinates to CSRS using the algorithm for that process described above.  Thus, we actually need to apply two distinct conversions in order to achieve the sanctioned results.

You can instruct the Coordinate Conversion System that this is what you desire/require by making an admittedly strange entry into the Nad27ToCsrs.gdc Geodetic Data Catalog file.  You add a reference to the Ats77ToCsrs.gdc file as if it were a grid shift file.

There are several other provinces which supply grid shift files to convert directly from NAD27 to CSRS (this saves users lots of extra processing).  Thus, you must use the positioning of the strange Ats77ToCsrs entry, and the parameters associated with it, to control the order of applicable data file search.  A simple rule to follow is to simply use a single entry in the Nad27ToCsrs.gdc Geodetic Data Catalog; that is the one you need to be applied for your current project.

See Also

Geodetic Data Catalog

Catalog Contents

Fallback Datum