GPS made WGS 84 the quiet default map of Earth
A geodetic datum is a reference frame for measuring positions on Earth—an ellipsoid plus orientation rules that turn latitude and longitude into shared meaning. Surveying, GIS, and navigation all lean on it. Since GPS rose, WGS 84 has replaced most local systems in everyday apps.
A datum has three ingredients: a model of Earth's shape, usually a reference ellipsoid or a geoid; an origin that pins the model to a known, often monumented spot, not necessarily at zero latitude and longitude; and a web of carefully surveyed control points measured out from that origin. Horizontal datums fix latitude and longitude, vertical datums fix height or depth against a standard such as mean sea level, and three-dimensional datums handle both together. Even Mars, which has no oceans and so no sea level, has had at least two datums.
The ancient Greeks knew Earth was round and devised latitude, longitude and astronomical ways to measure them, methods refined by Muslim and Indian astronomers and good enough for the voyages of the 15th and 16th centuries. The Enlightenment exposed their errors. John Harrison's marine chronometer of 1735 was one answer, and Isaac Newton argued that Earth should bulge at the equator. Triangulation surveys spread from Jacques Cassini's in 1718 and the Anglo-French survey of 1784 to 1790 to the Struve Geodetic Arc across Eastern Europe and the Great Trigonometrical Survey of India, which ran from 1802 to 1871.
In the United States this work produced NAD 27 and a 1929 vertical datum, the first standard datums open to public use. Satellites later sharpened the picture, yielding NAD 83 for North America, GDA94 for Australia and ETRS89 for Europe. The choice matters: the same point can shift by several hundred feet between NAD 27 and NAD 83, and Europe's ED50 of 1950 differs from WGS 84 by a few hundred metres depending on location.
GPS runs on WGS 84, a datum designed for the whole globe, and its spread has displaced most local systems. Local datums can still fit a region better, since Earth is not a perfect ellipsoid; OSGB36 hugs the British Isles more closely, for instance. For sea level, the gravity model EGM2008, built from at least 2,159 spherical harmonics, is far more precise than the WGS 84 ellipsoid, but the convenience of one worldwide frame usually wins.
Source: Geodetic datum
