GEOID

model of the figure of the Earth that coincides with mean sea level over the oceans and continues in continental areas as an imaginary sea-level surface defined by spirit level. It is everywhere perpendicular to the pull of gravity and approximates the shape of a regular oblate spheroid. It is irregular, however, because of local buried-mass concentrations (departures from lateral homogeneity at depth) and because of differences in elevation between continents and seafloors. Mathematically speaking, the geoid is an equipotential surface: that is, it is characterized by the fact that over its entire extent the potential function is constant. This potential function describes the combined effects of the gravitational attraction of the Earth's mass and the centrifugal repulsion caused by the rotation of the Earth about its axis.