Local vs. Earth-Centered Datums

A local datum aligns its ellipsoid to closely fit the earth's surface in a particular area. A point on the surface of the ellipsoid is matched to a particular position on the surface of the earth (the origin point of the datum). The coordinates of this point are fixed and the corodinates of all other points are calculated from it.

An Earth-Centered, or geocentric datum uses the Earth's center of mass as the origin point of the datum.

 

Comparing Common U.S. Datums

	Ellipsoid	Earth-Centered or Local?	Origin point of datum
NAD27	Clarke 1866	Local	Meade's Ranch, KS (coordinates determined using celestial measurements)
NAD83	GRS80	Earth-Centered (Geocentric)	Center of the Earth
WGS84	WGS84	Earth-Centered (Geocentric)	Center of the Earth

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