Earth Ellipsoid - Mean Earth Ellipsoid and reference Ellipsoids

Mean Earth Ellipsoid and reference Ellipsoids

A data set which describes the global average of the Earth's surface curvature is called the mean Earth Ellipsoid. It refers to a theoretical coherence between the geographic latitude and the meridional curvature of the geoid. The latter is close to the mean sea level, and therefore an ideal Earth ellipsoid has the same volume as the geoid.

While the mean Earth ellipsoid is the ideal basis of global geodesy, for regional networks a so called reference ellipsoid may be the better choice. When geodetic measurements have to be computed on a mathematical reference surface, this surface should have a similar curvature as the regional geoid. Otherwise the reduction of the measurements would get small distortions.

This is the reason for the "long life" of former reference ellipsoids like the Hayford or the Bessel ellipsoid, despite of the fact that their main axes deviate by several hundred meters from the modern values. Another reason is a juridical one: the coordinates of millions of boundary stones should remain fixed for a long period. If their reference surface would change, the coordinates themselves would also change.

However, for international networks, GPS positioning or astronautics, these regional reasons are less relevant. As the knowledge of Earth's figure is increasingly accurate, the International Geoscientific Union IUGG usually adopts the axes of the Earth ellipsoid to the best available data.