The geoid is a fundamental concept in geodesy representing an equipotential surface of Earth’s gravity field that closely approximates global mean sea level. Unlike the smooth mathematical ellipsoid used as a reference surface for GNSS coordinates, the geoid is an irregular, undulating surface shaped by the uneven distribution of mass within the Earth, including variations in crustal density, mountain ranges, ocean trenches, and deep mantle structures.
Understanding the geoid is essential for interpreting GNSS-derived heights and relating them to practical elevation concepts. GNSS receivers naturally determine positions relative to a mathematical ellipsoid (such as WGS84), producing ellipsoidal heights that have no direct physical meaning in terms of fluid flow or gravitational potential. The geoid, representing surfaces of constant gravitational potential, defines the direction water would flow, water naturally moves from higher to lower geoid heights, not necessarily from higher to lower ellipsoidal heights.
The separation between the ellipsoid and geoid surfaces at any location is called geoid undulation, geoid height, or geoid separation (typically denoted N). This value varies globally from approximately -107 meters to +85 meters, meaning the geoid surface can be either above or below the ellipsoid depending on location. Converting GNSS ellipsoidal heights (h) to orthometric heights (H) requires applying the geoid undulation: H = h – N.
Geoid models are mathematical representations of this complex surface, developed from satellite gravity measurements, terrestrial gravity surveys, and ocean altimetry data. Global models like EGM2008 and EGM2020 (Earth Gravitational Models) provide worldwide coverage, while national models such as GEOID18 (United States) or OSGM15 (United Kingdom) offer higher resolution and accuracy within their coverage areas. Proper application of appropriate geoid models is critical for any GNSS application requiring accurate elevations for engineering, construction, flood mapping, or scientific purposes.
