The shape of an object's gravitational equipotential surface. For the Earth, the reference geoid is
where is the colatitude. The most complete model for the earths gravitational field, based on an expansion in a Laplace series, Eric Weisstein's World of Math is given by the GEM-T2 model. It contains 600 coefficients above degree 36.
References
Marsh, J. G.; Lerch, F. J.; Putney, B. H.; Felsentreger, T. L.; Sanchez, B. V.; et al. "The GEM-T2 Gravitational Model." J. Geophys. Res. 95, B13, 22043-22071, 1990.
