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In geodesy and geophysics, theoretical gravity is a means to compare the true gravity on the Earth's surface with a physically smoothed model. The most common model of a smoothed Earth is the Earth ellipsoid.
Despite the fact that the exact density layers in the Earth's interior are still unknown, the theoretical gravity g of its level surface can be computed quite easily by using the International Gravity Formula. This refers to a mean Earth ellipsoid, the parameters of which are set by international convention. It shows the gravity at a smoothed Earth's surface as a function of geographic latitude φ; the actual formula is
Up to the 1960s, the formula either of the Hayford ellipsoid (1924) or of the famous German geodesist Helmert (1906) was used. Hayford has an axis difference [clarification needed] to modern values of 250 m, Helmert only 70 m. The Helmert formula is
A slightly different formula for g as a function of latitude is the WGS (World Geodetic System) 1984 Ellipsoidal Gravity Formula:
The difference between the WGS-84 formula and Helmert's equation is less than 0.68 ppm or 6.8×10−7 m·s−2.
- Karl Ledersteger: Astronomische und physikalische Geodäsie. Handbuch der Vermessungskunde Band 5, 10. Auflage. Metzler, Stuttgart 1969
- B.Hofmann-Wellenhof, Helmut Moritz: Physical Geodesy, ISBN 3-211-23584-1, Springer-Verlag Wien 2006.
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