Geoid

The geoid is the equipotential surface of the Earth's gravity field that best fits global mean sea level and extends beneath the continents. It represents the shape that would be formed by the surface of the oceans if they were influenced only by gravity and the Earth's rotation, without other forces such as currents or winds. Because gravity is not uniform, the geoid is irregular rather than a perfect ellipsoid.

In geodesy, the geoid serves as the reference surface from which elevations are measured. The difference between the geoid and a reference ellipsoid is called the geoid height, or N. Ellipsoidal height, H, obtained from GNSS (GPS) data, can be converted to orthometric height, h, using the relation H = h + N (or h = H − N). This enables accurate vertical positioning and the integration of diverse height systems.

Geoid undulations, the deviations of the geoid from the reference ellipsoid, arise from anomalies in Earth's mass distribution and from ocean mass variations. They can reach tens of meters locally, with global variations typically on the order of ±100 meters.

Determination of the geoid combines terrestrial gravity measurements, airborne and satellite gravity data, and sea-surface height measurements. Modern gravity field models derived from missions such as GOCE and GRACE, along with Stokes-type integrals and global gravimetric grids, are used to compute geoid models at high spatial resolution.

Applications of the geoid include precise leveling, surveying, construction, navigation, and studies of ocean circulation and sea-level change. It provides a physically meaningful reference surface for vertical positioning and for understanding Earth's gravity field.