A geodetic earth reference model is defined as a self-gravitating body of given mass and rotational rate whose surface is an equipotential ellipsoid of revolution of specified dimensions. Over the course of this century, the International Association of Geodesy has sanctioned three such models as recommended standards for both scientific and practical applications. The most recent model was approved in December 1979, replacing one chosen in 1967, which in turn supplanted another originally adopted four decades earlier.
One of the chief scientific aims of geodesy is the determination of the size and shape of the earth. Therefore, much of the energy expended by geodesists has focused on finding a model (defined by both geometrical and physical parameters) that can serve as a suitable reference surface for further geodetic and geophysical investigations. ‘Suitable’ is a flexible term. In the 19th century, it may have inferred accuracy to one part in 104 . Nowadays, it signifies better than one part in 106.