Earth ellipsoid

An Earth ellipsoid is a mathematically defined, slightly flattened ellipsoid that approximates Earth’s shape for geodetic measurements and mapping.

Oblate spheroid: A rotational ellipsoid whose polar axis is shorter than its equatorial diameter. Earth ellipsoids model Earth as an oblate spheroid, flattened at the poles.
Geodesy: The science of measuring Earth’s shape, orientation, gravity field, and changing positions. Geodesy estimates and applies ellipsoids as reference surfaces for measurement.
World Geodetic System 1984: A global geodetic reference system used by GPS and many mapping applications. Its reference ellipsoid supports worldwide positioning and coordinate conversion.
Sphere: A three-dimensional surface whose points are all equally distant from its center. A sphere ignores polar flattening that Earth ellipsoids explicitly model.
Geodetic datum: A reference framework that defines coordinates and positions on or near Earth. A datum anchors an ellipsoid to Earth and specifies how coordinates relate to it.
Geodetic coordinates: A position expressed by geodetic latitude, longitude, and height relative to an ellipsoid. These coordinates locate points using the ellipsoid as their geometric reference.
GRS 80: A geodetic reference ellipsoid adopted for systems including NAD83 and European terrestrial reference frames. It provides a widely used ellipsoid for regional and continental geodetic coordinates.
Topographic surface: The irregular physical boundary formed by Earth’s land, ice, and ocean surfaces. The topographic surface contains terrain that a reference ellipsoid deliberately smooths away.
Flattening: A measure of how much an ellipsoid’s polar axis differs from its equatorial diameter. Flattening quantifies the slight polar compression built into Earth ellipsoids.
Geoid: An equipotential surface of Earth’s gravity field that approximately matches global mean sea level. Unlike the geometric ellipsoid, the geoid follows variations in Earth’s gravity field.

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