geoidal

Geoidal is an adjective used in geodesy and related fields to describe things pertaining to the geoid, the theoretical surface representing the shape the oceans would take under Earth's gravity and rotation if not affected by winds, tides, or currents. The geoid is an equipotential surface of the Earth's gravity field and serves as a global reference for elevations. Because Earth's gravity varies with mass distribution, the geoid is irregular, deviating from a smooth ellipsoid by up to roughly ±100 meters in places. The difference between the geoid and a reference ellipsoid at a given location is called the geoid height or geoidal undulation.

Geoidal data are derived from gravimetric measurements and satellite missions that measure gravity anomalies, such as GRACE and GOCE, and from satellite altimetry. Global gravity models (for example EGM2008) and regional geoid models provide grids of geoid undulations used to convert between ellipsoidal heights (as measured by GNSS) and orthometric heights (as used in traditional height systems).

In practice, geoidal models underpin vertical datums, geodetic surveying, and oceanography. They enable accurate height determination for mapping, construction, and navigation, and they are essential for calibrating sea-level measurements and for comparative studies of gravity fields. The term geoidal is commonly used to describe phenomena, data, and models related to the geoid, its height above reference ellipsoids, and its irregular shape.