The visualization NASA published shows exaggerated bulges and depressions representing variations in gravitational pull, not physical terrain. It uses a scientific model known as the geoid to make subtle differences visible. The geoid is a gravity-based reference surface that approximates global mean sea level, not a map of mountains and ocean trenches or an image of Earth's solid surface.
Earth can be described as nearly spherical or as an oblate spheroid, because rotation slightly flattens the poles and bulges the equator. The geoid accounts for variations in gravity. Spain's Instituto Geográfico Nacional (IGN) explains the rotational effect. The ellipsoid captures Earth's broad geometry, but uneven mass distribution affects gravity from place to place. Mountains and ocean basins contribute to those differences, as does subsurface geology. NASA's visualization is based on the GOCO06s global gravity model, which represents the field mathematically rather than showing the planet's terrain.
The geoid is an equipotential surface, with gravity acting perpendicular to it, which makes it useful as a vertical reference. It approximates the level the oceans would take under the influence of gravity and Earth's rotation, with temporary effects such as winds and currents set aside, as are waves. NOAA describes the geoid as a model of global mean sea level used to define zero elevation. Because the gravity field is not uniform, this reference surface is irregular, though its departures are subtle in physical scale.
NASA exaggerates the variations in the image by a factor of 10,000 to make them visible. In reality, deviations from the mathematical ellipsoid, the standard geometric approximation of Earth, rarely exceed a few hundred meters. The enlarged relief shows variations in gravitational potential. It does not mean Earth's surface physically has such prominent bulges and depressions.
The geoid also has practical applications in surveying and mapping. It supports satellite positioning, too. GNSS receivers provide ellipsoidal height, while maps and engineering projects commonly use orthometric height, measured relative to the geoid and commonly described as height above mean sea level. The relationship is expressed as h = H + N, where h is ellipsoidal height, H is orthometric height and N is geoid height relative to the reference ellipsoid. Accurate geoid models help convert satellite positioning results into elevations used in construction and topographic mapping. Flood-risk assessments use them as well.
The GOCO06s model is a degree-300 global geopotential model. Its spherical-harmonic representation can be used to calculate geoid height and gravity disturbance, among related quantities. Technical documentation reports that an independent implementation reproduced the model with agreement of approximately 10⁻¹⁵ in potential and 10⁻⁶ microgal in gravity. These figures describe computational agreement, not the size of the geoid's physical undulations. The model behind NASA's visualization was developed using a billion satellite observations collected over 15 years by 19 satellites, including NASA's GRACE and the European Space Agency's GOCE missions.
Understanding the geoid is basic to geodesy, the science of Earth's shape and dimensions. Gravity-field models support precise elevation measurement and the interpretation of satellite observations. Changes in gravity can also reflect the redistribution of water and ice, along with other mass on Earth. In Spain, as in other countries, geodetic models support elevation data and infrastructure planning. International satellite missions continue to refine gravity models, improving measurements used in maps and navigation. Environmental monitoring also relies on those measurements.
