Not to be confused with geode, a crystal-filled rock.
Not to be confused with GEOID, a geocoding scheme.
Geodesy
Fundamentals
Geodesy
Geodynamics
Geomatics
History
Concepts
Geographical distance
Geoid
Figure of the Earth (radius and circumference)
Geodetic coordinates
Geodetic datum
Geodesic
Horizontal position representation
Latitude / Longitude
Map projection
Reference ellipsoid
Satellite geodesy
Spatial reference system
Spatial relations
Vertical positions
Technologies
Global Nav. Sat. Systems (GNSSs)
Global Pos. System (GPS)
GLONASS (Russia)
BeiDou (BDS) (China)
Galileo (Europe)
NAVIC (India)
Quasi-Zenith Sat. Sys. (QZSS) (Japan)
Discrete Global Grid and Geocoding
Standards (history)
NGVD 29
Sea Level Datum 1929
OSGB36
Ordnance Survey Great Britain 1936
SK-42
Systema Koordinat 1942 goda
ED50
European Datum 1950
SAD69
South American Datum 1969
GRS 80
Geodetic Reference System 1980
ISO 6709
Geographic point coord. 1983
NAD 83
North American Datum 1983
WGS 84
World Geodetic System 1984
NAVD 88
N. American Vertical Datum 1988
ETRS89
European Terrestrial Ref. Sys. 1989
GCJ-02
Chinese obfuscated datum 2002
Geo URI
Internet link to a point 2010
International Terrestrial Reference System
Spatial Reference System Identifier (SRID)
Universal Transverse Mercator (UTM)
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The geoid (/ˈdʒiː.ɔɪd/JEE-oyd) is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extended through the continents (such as might be approximated with very narrow hypothetical canals). According to Gauss, who first described it, it is the "mathematical figure of the Earth", a smooth but irregular surface whose shape results from the uneven distribution of mass within and on the surface of Earth.[1] It can be known only through extensive gravitational measurements and calculations. Despite being an important concept for almost 200 years in the history of geodesy and geophysics, it has been defined to high precision only since advances in satellite geodesy in the late 20th century.
The geoid is often expressed as a geoid undulation or geoidal height above a given reference ellipsoid, which is a slightly flattened sphere whose equatorial bulge is caused by the planet's rotation. Generally the geoidal height rises where the Earth's material is locally more dense and exerts greater gravitational force than the surrounding areas. The geoid in turn serves as a reference coordinate surface for various vertical coordinates, such as orthometric heights, geopotential heights, and dynamic heights (see Geodesy#Heights).
All points on a geoid surface have the same geopotential (the sum of gravitational potential energy and centrifugal potential energy). At this surface, apart from temporary tidal fluctuations, the force of gravity acts everywhere perpendicular to the geoid, meaning that plumb lines point perpendicular and bubble levels are parallel to the geoid.
Being an equigeopotential means the geoid corresponds to the free surface of water at rest (if only the Earth's gravity and rotational acceleration were at work); this is also a sufficient condition for a ball to remain at rest instead of rolling over the geoid.
Earth's gravity acceleration (the vertical derivative of geopotential) is thus non-uniform over the geoid.[2]
Geoid undulation in pseudocolor, shaded relief and vertical exaggeration (10000 vertical scaling factor).Geoid undulation in pseudocolor, without vertical exaggeration.
^Gauß, C.F. (1828). Bestimmung des Breitenunterschiedes zwischen den Sternwarten von Göttingen und Altona durch Beobachtungen am Ramsdenschen Zenithsector (in German). Vandenhoeck und Ruprecht. p. 73. Retrieved 6 July 2021.
^Geodesy: The Concepts. Petr Vanicek and E.J. Krakiwsky. Amsterdam: Elsevier. 1982 (first ed.): ISBN 0-444-86149-1, ISBN 978-0-444-86149-8. 1986 (third ed.): ISBN 0-444-87777-0, ISBN 978-0-444-87777-2. ASIN 0444877770.
The geoid (/ˈdʒiː.ɔɪd/ JEE-oyd) is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction...
Grid Cells NAC (patended), area codes (area can be indefinitely small) GEOID, the name of United States Census Bureau geographic identifiers. In the...
between the two, referred to as geoid undulations, geoid heights, or geoid separations, will be irregular as well. The geoid is a surface along which the...
the study of Earth's gravitational field is called physical geodesy. The geoid essentially is the figure of Earth abstracted from its topographical features...
and gravimetric geoids. By matching the relative astro-geodetic geoids of the selected datums with an earth-centered gravimetric geoid, the selected datums...
The Indian Ocean Geoid Low (IOGL) is a gravity anomaly in the Indian Ocean. A circular geoid low situated just south of the Indian peninsula, it is the...
ellipsoid. This difference is the geoid height, positive above or outside the ellipsoid, negative below or inside. The geoid height variation is under 110 m...
anomalies, defines the geoid of the Earth, which approximates the local mean sea level for locations in the open ocean. The geoid includes a significant...
approximate the geoid by a mathematically simpler reference surface. The simplest choice for the reference surface is a sphere, but the geoid is more accurately...
is a planetary geoid that represents the gravitational and rotational equipotential figure of Mars, analogous to the concept of geoid ("sea level") on...
are measured from is called the quasi-geoid (or quasigeoid), a representation of mean sea level similar to the geoid and close to it, but lacking the physical...
geometric reference frame and geopotential datum, based on GPS and gravimetric geoid models. NAVD 88 was established in 1991 by the minimum-constraint adjustment...
undefined value: it refers to an altitude of 0 meters above the geoid. The use of a geoid stands in contrast to GeoJSON, which uses direct ellipsoid height...
Earth's shape can be thought of in at least two ways: as the shape of the geoid, the mean sea level of the world ocean; or as the shape of Earth's land...
ellipsoid surface) or altitude/elevation (above and below the geoid). The areoid (the geoid of Mars) has been measured using flight paths of satellite missions...
separation between the geoid and the reference ellipsoid is called the undulation of the geoid, symbol N {\displaystyle N} . The geoid, or mathematical mean...
represents the horizontal spatial gradient of the geoid undulations of the geoid (i.e., the separation between geoid and reference ellipsoid). In practice, the...
topography, geodesy employs an idealized Earth producing a shape called a geoid. Such a geoid shape is gained if the ocean is idealized, covering Earth completely...
and dimensions, such as a reference ellipsoid or a geoid; an origin at which the ellipsoid/geoid is tied to a known (often monumented) location on or...
Fundamentals Geodesy Geodynamics Geomatics History Concepts Geographical distance Geoid Figure of the Earth (radius and circumference) Geodetic coordinates Geodetic...
and their proper motions precise astronomical navigation astro-geodetic geoid determination modelling the rock densities of the topography and of geological...
the National Geospatial-Intelligence Agency (NGA). They are used as the geoid reference in the World Geodetic System. The NGA provides the models in two...
linear scaling of TCG, such that the unit of TT is the "SI second on the geoid", i.e. the rate approximately matched the rate of proper time on the Earth's...
extremes by region. Elevation is the vertical distance above the reference geoid, an equipotential gravitational surface model of the Earth's sea level....
a datum, that is an approximated a "standard model of the Geoid". So, in the role of Geoid, the "globe" covered by a DGG can be any of the following objects:...
geoid-ellipsoid separation, N. It varies globally between ±110 m. A reference ellipsoid, customarily chosen to be the same size (volume) as the geoid...
positioning, and navigation (geometric satellite geodesy): 3 Determination of geoid, Earth's gravity field and its temporal variations (dynamical satellite...
curvature of the geoid. The latter is close to the mean sea level, and therefore an ideal Earth ellipsoid has the same volume as the geoid. While the mean...
territory. Topographic elevation is the vertical distance above the reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational...
1929 was a hybrid model, it was not a pure model of mean sea level, the geoid, or any other equipotential surface. Therefore, it was renamed the National...
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