Geoid

Overview The geoid is the hypothetical surface of the Earth's gravity field where gravitational potential is everywhere equal, corresponding approximately to mean sea level extended continuously through the continents. Unlike the smooth mathematical ellipsoid, the geoid has an irregular shape that reflects variations in Earth's internal mass distribution, with undulations of up to 100 meters above and below the reference ellipsoidReference EllipsoidA reference ellipsoid is a mathematically defined surface that approximates the shape of the Earth as an oblate spher.... The geoid is the natural reference surface for measuring physical heights. Geoid vs. Ellipsoid The relationship between the geoid and the reference ellipsoid is critical for height determination. GPSGPSThe Global Positioning System (GPS) is a satellite-based navigation system operated by the U.S. Space Force that prov... and GNSSGNSSGlobal Navigation Satellite System (GNSS) is the umbrella term for satellite-based positioning systems that provide a... systems measure heights relative to the ellipsoid (ellipsoidal heights), but practical applications like engineering, hydrology, and mapping require orthometric heights relative to the geoid (effectively, heights above mean sea level). Converting between these requires a geoid model that specifies the geoid undulation (separation) at any location. Models like EGM2008 (Earth Gravitational Model 2008) provide this information globally at high resolution.

Geoid models are computed from a combination of satellite gravity observations (from missions like GRACE and GOCE), surface gravity measurements, and topographic data. Global models like EGM2008 achieve centimeter-level accuracy in well-surveyed regions. Regional geoid models developed by national geodetic agencies provide higher accuracy for specific countries or regions. These models are essential components of vertical datumDatumA geodetic datum is a mathematical model that defines the size, shape, and orientation of the Earth, serving as the r... definitions and are regularly refined as new observational data becomes available.

Geoid models are used by GNSSGNSSGlobal Navigation Satellite System (GNSS) is the umbrella term for satellite-based positioning systems that provide a... receivers and post-processing software to convert ellipsoidal heights to orthometric heights for surveying and engineering applications. Flood modelingFlood ModelingFlood Modeling uses hydrological simulation and geospatial analysis to predict the extent, depth, and velocity of flo... requires geoid-referenced elevations for accurate water surface prediction. Sea level monitoring uses the geoid as the reference for detecting long-term trends. Geophysical research analyzes geoid anomalies to study Earth's internal structure and mass distribution.