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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.

Datum Comparison

 

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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