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Datums and Ellipsoids: NAD83, WGS84, and GRS80

Question asked by Vince Edwards on Sep 4, 2018
Latest reply on Feb 13, 2020 by Rhett Wheeler

In my version of GPS Pathfinder Office (5.4), in the Software Help window under "About Coordinate Systems and Datums," I find the following text:


"Various datums have been established that best suit particular regions. For example, maps in the United States are often based on the North American Datum of 1927 or 1983 (NAD-27, NAD-83). The only global datum is WGS-84, which is based on the GRS-80 ellipsoid. All GNSS coordinates are based on the WGS-84 datum surface."


I find this to be a little confusing. My understanding is that the NAD-83 datum is based on (or "uses," whatever the proper word there would be) the GRS-80 ellipsoid, and the WGS-84 datum is based on its own ellipsoid of the same name (WGS-84). I know these two ellipsoids are remarkably close, however they are absolutely not the same thing. Is Pathfinder simplifying things by saying that the WGS-84 ellipsoid is based on the GRS-80 ellipsoid?


Thanks in advance if anyone can add some clarity.