A representation of the earth's surface if it was completely covered by oceans. This representation is also known as the surface of equal gravitational potential, which is essentially mean sea level. A geoid is an ellipsoid with variations that account for landmass, ocean trenches, and gravitational and centrifugal forces. It is irregular and varies from a perfect sphere by as much as 75 meters above and 100 meters below its surface. Used in geolocation, object, and seismic measurement applications.