GEOG 862
GPS and GNSS for Geospatial Professionals

Modern Geocentric Datum

Two Reference Ellipsoids
Ellipsoid Semi-major axis inverse flattening
GRS80 6,378,137 m 298.257222101
WGS84 6,378,137 m 298.257223563


In the table above, you have a comparison between WGS84 and GRS80 ellipsoids. You will notice that the semi major axis is the same, and the inverse flattening is only different when you get out to the sixth place to the right of the decimal. They are very similar. The WGS 84 ellipsoid is the reference of the datum known as the World Geodetic System 1984 (WGS84). In other words, the WGS 84 ellipsoid attached at the center of mass of the Earth is one component of the WGS84 datum, but please note that while the WGS84 ellipsoid is the reference ellipsoid for the WGS84 datum - it is not the datum itself.

The WGS84 datum (reference frame) has been used by the U.S. Military since January 21, 1987. There have been six incarnations of WGS84 since then. While WGS84 has always been the basis for the GPS Navigation message, the particular version of the datum has changed. As of this writing, the latest version of WGS84 is WGS84 (G1762). The number following the letter G is the number of the GPS week during which the coordinates first were used in the National Geospatial Intelligence Agency, NGA, precise ephemeris estimations. Therefore, coordinates provided today by GPS receivers are based in WGS84 (G1762) which is the sixth update to the realization of the WGS84 Reference Frame. The original WGS 84 was based on observations from more than 1900 Doppler Stations. It was revised to become WGS84 (G730) to incorporate GPS observations. That realization was implemented in GPS by the Operational Control Segment, OCS on June 29, 1994. More GPS based realizations of WGS84 followed, WGS84 (G873) on January 29, 1997, and WGS84 (G1150) was implemented on January 20, 2002 and WGS84(G1674) on February 8, 2012. Today, the epoch of WGS84 is (G1762).

Most available GPS software can transform those coordinates to a number of other datums as well. The one that is probably of greatest interest in the United States today is the North American Datum 1983 (NAD83). Originally, the difference between WGS84 as originally rolled out in 1987 and NAD83 as first introduced in 1986 coordinates was so small that transformation was unnecessary. That is no longer the case when it comes to NAD83 2011 (2010.0) and WGS84 (G1762) the difference can be ~1 to 2 meters.