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Geoid

The gravitational potential of a planet is determined by the mass distribution within it (see Gravity fields of the terrestrial planets). Any non-radially symmetric density distribution will produce irregularly shaped equipotential or level surfaces.

In the absence of winds, currents and other oceanographic perturbations, the Earth's ocean surface is one such surface and, as such, it provides a natural definition for the shape of the Earth. This is the geoid; defined as that equipotential surface corresponding to the mean sea surface over the oceans and, on the continents, as the surface that would be defined by 'a series of criss-crossing free-flowing canals in open connection to the sea'.

Because of its rotation, the Earth is permanently deformed and the geoid can, in a first approximation, be represented by an ellipsoid of revolution, with its short axis along the rotation axis, and with a geometric flattening f of
where R e and