International Gravity Formula
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As a first approximation, the Earth is a rotating sphere. As a second approximation, it can be regarded as an equipotential ellipsoid of revolution. According to Moritz (1980), the theory of this equipotential ellipsoid was first given by P. Pizzetti in 1894. It was further elaborated by C. Somigliana in 1929 and served already as the basis for the International Gravity Formula adopted at the General Assembly of the International Union of Geodesy and Geophysics (IUGG) in Stockholm in 1930.
One particular ellipsoid of revolution, also called the “normal Earth” or “normal spheroid,” is the one having the same angular velocity and the same mass as the actual Earth, the potential U 0 on the ellipsoid surface equal to the potential W 0 on the geoid, and the center coincident with the center of mass of the Earth. The Geodetic Reference System 1967 (GRS 67), Geodetic Reference System 1980 (GRS 80), and World Geodetic System 1984 (WGS 84) all are “normal Earth” models.