Geodesy

Geodesy is the science of measuring Earth’s shape, its gravity field, and its orientation in space, and how these change over time. It defines the reference ellipsoid and the geoid, builds the datums and coordinate frames that all positioning depends on, and resolves the difference between ellipsoidal and orthometric height through the geoid undulation. Modern geodesy uses GNSS, satellite altimetry, very-long-baseline interferometry, and satellite gravity missions such as GRACE to monitor plate motion, sea-level rise, and crustal deformation at millimeter precision. Hydrography rests on geodesy: every sounding position and chart datum is tied to a geodetic frame, normally WGS 84.