Geoiden

The geoid is the equipotential surface of Earth's gravity field that best corresponds to mean sea level worldwide. It is a theoretical, irregular surface that stretches beneath the oceans and through the continents. Because Earth's mass distribution and topography are uneven, the geoid undulates with elevation relative to a simple reference shape. The geoid is not a physical sheet; gravity follows it, and the direction of gravity is locally perpendicular to the surface. In German geodesy, the term Geoide is used for this concept.

Geodesy uses the geoid as the reference surface for height measurements. Orthometric height H represents height above the geoid, while ellipsoidal height h is measured from a reference ellipsoid (such as WGS84). The geoid height, or undulation N, is the difference between the geoid and the ellipsoid at a given location (N = h − H).

Geoid models are generated from gravity data collected on land and at sea, plus satellite gravity missions such as GOCE and GRACE, and are combined with GNSS measurements to produce regional or global models (for example, EGM2008, EGM2020). These models are essential for accurate surveying, mapping, navigation, precise positioning, and studies of sea-level change.