Reference ellipsoid

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Arbitrary reference surface that is a raw approximation of the Earth’s shape, which is basically a sphere “flattened” at its poles. The length of one of the axes at the Equator is chosen so that the ellipsoid coincides at this latitude with the mean sea level. For example, the ellipsoid used by the Topex/Poseidon mission has a radius of 6378.1363 km and a flattening of 1/298.257.