Other meanings of Geodesy
Earth science
Geodesy is the scientific study of Earth’s shape, orientation, and gravity field. It provides the measurements, reference frames, and models needed to locate points consistently on a changing planet, from continental-scale surveying to satellite navigation and monitoring sea-level change.
Geodesy describes Earth through interrelated measurements of geometry, rotation, and gravity. Because the planet is irregular, it is represented by several reference surfaces rather than by a perfect sphere. The ellipsoid is a mathematically convenient approximation of Earth’s broad shape, while the geoid is an equipotential surface of the gravity field that roughly coincides with global mean sea level.1
Modern geodesy distinguishes ellipsoidal height, measured from a reference ellipsoid, from orthometric height, commonly associated with elevation above the geoid. That distinction explains why satellite-derived heights and conventional levelling can differ unless they are connected through a geoid model. Geodesists also account for crustal motion, tides, atmospheric pressure, ocean loading, and mass redistribution, all of which alter measured positions or the underlying field.
Geodetic reference frames make positions comparable across countries, instruments, and decades. A frame specifies coordinates, an origin, scale, orientation, and the conventions used to realize them; the World Geodetic System 1984 is widely used for satellite positioning, while the International Terrestrial Reference Frame provides a scientific realization tied to a global network of observations.
Measurements combine several techniques. Global Navigation Satellite Systems determine three-dimensional positions, very-long-baseline interferometry links distant radio telescopes, satellite laser ranging measures distances to orbiting targets, and Doppler Orbitography and Radiopositioning Integrated by Satellite contributes precise satellite-orbit information. Gravimeters, levelling, tide gauges, and satellite altimetry supply complementary information about height and gravity. Earth-orientation parameters, including polar motion and variations in the length of day, are maintained through international observations.
Geodesy underpins mapping, navigation, engineering, and the detection of environmental change. Survey control networks help construct roads, bridges, tunnels, dams, and buildings; cadastral systems use coordinates to define land parcels; and navigation services depend on a consistent terrestrial reference frame. Precise positioning also supports earthquake studies by revealing permanent or transient crustal displacements.1
Gravity and height observations extend this role into Earth-system science. Satellite missions such as GRACE detect changes in Earth’s gravity caused by movement of water and ice, allowing estimates of groundwater loss, ice-sheet mass change, and large-scale hydrological variation.2 Satellite altimetry measures sea-surface height, but interpreting it requires geodetic orbit determination, gravity models, and corrections for tides and atmospheric effects.
Geodetic coordinates are not permanently fixed because Earth’s surface and rotation change. Tectonic plates move centimetres per year, postglacial rebound slowly reshapes parts of the crust, and earthquakes can shift reference stations by metres in seconds. A coordinate therefore requires an epoch, or date, as well as a reference frame; ignoring the epoch can produce significant errors in long-term surveying and scientific comparisons.
Geodesy also reaches beneath and above the visible surface. Airborne and terrestrial gravimetry can reveal buried geological structures, while geoid undulations reflect variations in the density of rocks and ocean depths. The field has an overlooked historical connection to metrology: measurements of meridian arcs helped define early length standards, and today precise geodesy remains linked to international timekeeping, atomic clocks, and relativistic corrections in satellite navigation.
Geodetic quantities are convention-dependent: a coordinate, height, or gravity value should be interpreted together with its reference frame, realization, epoch, and measurement method.
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