# Reference ellipsoid

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In geodesy, a reference ellipsoid is a mathematically-defined surface that approximates the geoid, the truer figure of the Earth, or other planetary body. Because of their relative simplicity, reference ellipsoids are used as a preferred surface on which geodetic network computations are performed and point coordinates such as latitude, longitude, and elevation are defined.

--Source Wikipedia

Typically the reference ellipsoid is an oblate spheroid; its dimensions chosen so that the ellipsoid approximates the volume of the geoid. When used for geodetic calculations, the reference ellipsoid is known as a horizontal datum.