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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 1Illustrated map (from Cartography)
Image 2Datum shift between NAD27 and NAD83, in metres (from Geodesy)
Image 3GPS Block IIA satellite orbits over the Earth. (from Geodesy)
Image 4The cartographic process (from Cartography)
Image 5How very-long-baseline interferometry (VLBI) works (from Geodesy)
Image 6Ellipsoid - a mathematical representation of the Earth. When mapping in geodetic coordinates, a latitude circle forms a truncated cone. (from Geodesy)
Image 7Gravity measurement devices, pendulum (left) and absolute gravimeter (right) (from Geodesy)
Image 8Mapping 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 9Earth's rotation imaged by Deep Space Climate Observatory, showing axistilt (from Earth's rotation)
Image 10A 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 11On 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 12The Bedolina Map and its tracing, 6th–4th century BCE (from Cartography)
Image 13Valcamonica rock art (I), Paspardo r. 29, topographic composition, 4th millennium BCE (from Cartography)
Image 14A Munich archive with lithography plates of maps of Bavaria (from Geodesy)
Image 15Navigation device, Apollo program (from Geodesy)
Image 16Surveyor using a total station (from Geomatics)
Image 17Earth'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 18Relief map Sierra Nevada (from Cartography)
Image 19Deviation of day length from SI-based day (from Earth's rotation)
Image 20Axial 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 212D grid for elliptical coordinates (from Geodesy)
Image 22A relative gravimeter (from Geodesy)
Image 23Earth'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 24Plot 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 25Geoid, an approximation for the shape of the Earth; shown here with vertical exaggeration (10000 vertical scaling factor). (from Geodesy)
Image 26A modern instrument for geodetic measurements using satellites (from Geodesy)
Image 27The 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 28Gravity 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 29This 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 30A 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 31A 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 32Principles of geolocation using GPS (from Geopositioning)
Image 33Areal distortion caused by Mercator projection (from Cartography)
Image 34Topographic map of Easter Island (from Cartography)
Image 35Equatorial ( $a$ ), polar ( $b$ ) and mean Earth radii as defined in the 1984 World Geodetic System (from Geodesy)
Image 36Visual fix by three bearings plotted on a nautical chart (from Geopositioning)
Earth's gravity vs. distance from it, from the surface to 30000 km (from Gravity of Earth)
Image 38Height measurement using satellite altimetry (from Geodesy)
Image 39Variations in the gravity field of the Moon, from NASA (from Geodesy)
Image 40The Tabula Rogeriana, drawn by Muhammad al-Idrisi for Roger II of Sicily in 1154. South is at the top. (from Cartography)
Image 41A plumb bob determines the local vertical direction (from Gravity of Earth)
Image 42Small section of an orienteering map (from Cartography)
Image 43An artist's rendering of the protoplanetary disk (from Earth's rotation)
Image 44A simulated history of Earth's day length, depicting a resonant-stabilizing event throughout the Precambrian era (from Earth's rotation)
Image 45Starry circles arc around the south celestial pole, seen overhead at ESO's La Silla Observatory. (from Earth's rotation)
Image 46Europa regina in Sebastian Münster's "Cosmographia", 1570 (from Cartography)
Image 47Copy (1472) of St. Isidore's TO map of the world. (from Cartography)
Image 48A 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 49Geodetic control mark (from Geodesy)
Image 50Global gravity anomaly animation over oceans from the NASA's GRACE (Gravity Recovery and Climate Experiment) (from Geodesy)
Image 51Global plate tectonic movement using GPS (from Geodesy)

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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
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