 Theoretical gravity

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 of 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 by a relative simple formula, which is called the International Gravity Formula. It 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
The term 0.0516323 is called gravity flattening (abbreviated β). As a physically defined form parameter it corresponds to the geometrical flattening f of the earth ellipsoid.
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 WGS84 formula and Helmert's equation is less than 0.68 ppm or 6.8×10^{−7} m·s^{−2}.
See also
Literature
 Karl Ledersteger: Astronomische und physikalische Geodäsie. Handbuch der Vermessungskunde Band 5, 10. Auflage. Metzler, Stuttgart 1969
 B.HofmannWellenhof, Helmut Moritz: Physical Geodesy, ISBN 3211235841, SpringerVerlag Wien 2006.
Categories: Gravimetry
 Geophysics stubs
Wikimedia Foundation. 2010.