Defined through a consistent set of station coordinates. These have changed with time: by 0.7m on 29/6/1994 [WGS 84 (G730)], a further 0.2m on 29/1/1997 [WGS 84 (G873)] and a further 0.06m on 20/1/2002 [WGS 84 (G1150)].