It refers to a mean Earth ellipsoid, the parameters of which are set by international convention.

The most common model of a smoothed Earth is the Earth ellipsoid.

The latter is close to the mean sea level, and therefore an ideal Earth ellipsoid has the same volume as the geoid.

The reference ellipsoid for Earth is called an Earth ellipsoid.

A data set which describes the global average of the Earth's surface curvature is called the mean Earth Ellipsoid.

As a physically defined form parameter it corresponds to the geometrical flattening f of the earth ellipsoid.

Earth ellipsoid WGS84 (defined directly by a and ƒ):

Later grade measurements aimed at determining the flattening of the Earth ellipsoid by measuring at different geographic latitudes.

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.

An Earth ellipsoid is a mathematical figure approximating the shape of the Earth, used as a reference frame for computations in geodesy, astronomy and the geosciences.

The Earth is roughly spherical, but it bulges towards the Equator, so it is roughly in the shape of an ellipsoid (see Earth ellipsoid).

Many methods exist for determination of the axes of an Earth ellipsoid, ranging from meridian arcs up to modern satellite geodesy or the analysis and interconnection of continental geodetic networks.

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.

Therefore it is optimal for National survey networks in these regions, despite of the fact that its axes are about 700 m shorter than that of the mean Earth ellipsoid derived by satellites.

EPSG compiled and disseminated the EPSG Geodetic Parameter Set, a widely used database of Earth ellipsoids, geodetic datums, geographic and projected coordinate systems, units of measurement, etc.

In any case, the geographic latitude and longitude of the station is precisely determined by methods of astrogeodesy and is chosen as ellipsoidal latitude and longitude at the Earth ellipsoid which is used to calculate the coordinates of the whole network.