The geoid is often expressed as a geoid undulation or geoidal height above a given reference ellipsoid, which is a slightly flattened sphere whose equatorial bulge is caused by the planet's rotation. Generally the geoidal height rises where the Earth's material is locally more dense and exerts greater gravitational force than the surrounding areas. The geoid in turn serves as a reference coordinate surface for various vertical coordinates, such as orthometric heights, geopotential heights, and dynamic heights (see Geodesy#Heights).
the geoid is often expressed as a geoid undulation or geoidal height above a given reference ellipsoid which is a slightly flattened sphere whose equatorial bulge is caused by the planet's rotation generally the geoidal height rises where the earth's material is locally more dense and exerts greater gravitational force than the surrounding areas the geoid in turn serves as a reference coordinate surface for various vertical coordinates such as orthometric heights geopotential heights and dynamic heights (see geodesy#heights)