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The Geodesy Portal

GPS Block II-F satellite in Earth orbit

Geodesy or geodetics is the science of measuring and representing the geometry, gravity, and spatial orientation of the Earth in temporally varying 3D. It is called planetary geodesy when studying other astronomical bodies, such as planets or circumplanetary systems. Geodesy is an earth science and many consider the study of Earth's shape and gravity to be central to that science. It is also a discipline of applied mathematics.

Geodynamical phenomena, including crustal motion, tides, and polar motion, can be studied by designing global and national control networks, applying space geodesy and terrestrial geodetic techniques, and relying on datums and coordinate systems. Geodetic job titles include geodesist and geodetic surveyor. (Full article...)

Selected images

Image 1GPS Block IIA satellite orbits over the Earth. (from Geodesy)
Image 2Ellipsoid - a mathematical representation of the Earth. When mapping in geodetic coordinates, a latitude circle forms a truncated cone. (from Geodesy)
Image 3The definition of latitude (φ) and longitude (λ) on an ellipsoid of revolution (or spheroid). The graticule spacing is 10 degrees. The latitude is defined as the angle between the normal to the ellipsoid and the equatorial plane. (from Geodesy)
Image 4Geodetic control mark (from Geodesy)
Image 5The Bedolina Map and its tracing, 6th–4th century BCE (from Cartography)
Image 6Navigation device, Apollo program (from Geodesy)
Image 7A 14th-century Byzantine map of the British Isles from a manuscript of Ptolemy's Geography, using Greek numerals for its graticule: 52–63°N of the equator and 6–33°E from Ptolemy's Prime Meridian at the Fortunate Isles. (from Cartography)
Image 8A relative gravimeter (from Geodesy)
Image 9Europa regina in Sebastian Münster's "Cosmographia", 1570 (from Cartography)
Image 10A simulated history of Earth's day length, depicting a resonant-stabilizing event throughout the Precambrian era (from Earth's rotation)
Image 11Earth's gravity measured by NASA GRACE mission, showing deviations from the theoretical gravity of an idealized, smooth Earth, the so-called Earth ellipsoid. Red shows the areas where gravity is stronger than the smooth, standard value, and blue reveals areas where gravity is weaker (Animated version). (from Gravity of Earth)
Image 12Principles of geolocation using GPS (from Geopositioning)
Image 13Variations in the gravity field of the Moon, from NASA (from Geodesy)
Image 14How very-long-baseline interferometry (VLBI) works (from Geodesy)
Image 15Areal distortion caused by Mercator projection (from Cartography)
Image 16Visual fix by three bearings plotted on a nautical chart (from Geopositioning)
Image 17Mapping can be done with GPS and laser rangefinder directly in the field. Image shows mapping of forest structure (position of trees, dead wood and canopy). (from Cartography)
Image 18Geoid, an approximation for the shape of the Earth; shown here with vertical exaggeration (10000 vertical scaling factor). (from Geodesy)
Image 19Datum shift between NAD27 and NAD83, in metres (from Geodesy)
Image 20Global gravity anomaly animation over oceans from the NASA's GRACE (Gravity Recovery and Climate Experiment) (from Geodesy)
Image 21A plumb bob determines the local vertical direction (from Gravity of Earth)
Image 22A modern instrument for geodetic measurements using satellites (from Geodesy)
Image 23An artist's rendering of the protoplanetary disk (from Earth's rotation)
Image 24Equatorial ( $a$ ), polar ( $b$ ) and mean Earth radii as defined in the 1984 World Geodetic System (from Geodesy)
Image 25Surveyor using a total station (from Geomatics)
Image 26Topographic map of Easter Island (from Cartography)
Image 27Earth's rotation imaged by Deep Space Climate Observatory, showing axistilt (from Earth's rotation)
Image 28Gravity measurement devices, pendulum (left) and absolute gravimeter (right) (from Geodesy)
Image 29Global plate tectonic movement using GPS (from Geodesy)
Image 30This long-exposure photo of the northern night sky above the Nepali Himalayas shows the apparent paths of the stars as Earth rotates. (from Earth's rotation)
Image 312D grid for elliptical coordinates (from Geodesy)
Image 32Small section of an orienteering map (from Cartography)
Image 33Plot of latitude versus tangential speed. The dashed line shows the Kennedy Space Center example. The dot-dash line denotes typical airliner cruise speed. (from Earth's rotation)
Image 34A map of recent volcanic activity and ridge spreading. The areas where NASA GRACE measured gravity to be stronger than the theoretical gravity have a strong correlation with the positions of the volcanic activity and ridge spreading. (from Gravity of Earth)
Image 35Axial tilt (or Obliquity), rotation axis, plane of orbit, celestial equator and ecliptic. Earth is shown as viewed from the Sun; the orbit direction is counter-clockwise (to the left). (from Geodesy)
Image 36A medieval depiction of the Ecumene (1482, Johannes Schnitzer, engraver), constructed after the coordinates in Ptolemy's Geography and using his second map projection. The translation into Latin and dissemination of Geography in Europe, in the beginning of the 15th century, marked the rebirth of scientific cartography, after more than a millennium of stagnation. (from Cartography)
Image 37Illustrated map (from Cartography)
Earth's gravity vs. distance from it, from the surface to 30000 km (from Gravity of Earth)
Image 39Copy (1472) of St. Isidore's TO map of the world. (from Cartography)
Image 40Height measurement using satellite altimetry (from Geodesy)
Image 41Earth's axial tilt is about 23.4°. It oscillates between 22.1° and 24.5° on a 41,000-year cycle and is currently decreasing. (from Earth's rotation)
Image 42Valcamonica rock art (I), Paspardo r. 29, topographic composition, 4th millennium BCE (from Cartography)
Image 43Starry circles arc around the south celestial pole, seen overhead at ESO's La Silla Observatory. (from Earth's rotation)
Image 44Deviation of day length from SI-based day (from Earth's rotation)
Image 45Gravity at different internal layers of Earth (1 = continental crust, 2 = oceanic crust, 3 = upper mantle, 4 = lower mantle, 5+6 = core, A = crust-mantle boundary) (from Gravity of Earth)
Image 46A Munich archive with lithography plates of maps of Bavaria (from Geodesy)
Image 47The cartographic process (from Cartography)
Image 48The Tabula Rogeriana, drawn by Muhammad al-Idrisi for Roger II of Sicily in 1154. South is at the top. (from Cartography)
Image 49A pre-Mercator nautical chart of 1571, from Portuguese cartographer Fernão Vaz Dourado (c. 1520 – c. 1580). It belongs to the so-called plane chart model, where observed latitudes and magnetic directions are plotted directly into the plane, with a constant scale, as if the Earth were a plane (Portuguese National Archives of Torre do Tombo, Lisbon). (from Cartography)
Image 50On a prograde planet like Earth, the stellar day is shorter than the solar day. At time 1, the Sun and a certain distant star are both overhead. At time 2, the planet has rotated 360 degrees and the distant star is overhead again but the Sun is not (1→2 = one stellar day). It is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day). (from Earth's rotation)
Image 51Relief map Sierra Nevada (from Cartography)

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v t e Geodesy
Overview	History Geodesists
Subfields	Cartography Computer cartography Web mapping Earth's orbit Geodetic astronomy Geomatics Gravity of Earth Navigation Photogrammetry Remote Sensing Geopositioning Virtual globe
Physical phenomena	Chandler wobble Coriolis effect Earth's energy budget Earth's gravity field Geodynamo Gravity of Earth Plate tectonics Precession of the equinoxes Tide
Related disciplines	Astronomy Geology Geophysics Mathematics Physics
Geodesy portal Category Commons