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The geoid undulation (also known as geoid height or geoid anomaly), N, is the height of the geoid relative to a given ellipsoid of reference. N = h − H {\displaystyle N=h-H} The undulation is not standardized, as different countries use different mean sea levels as reference, but most commonly refers to the EGM96 geoid.
The orthometric height (symbol H) is the vertical distance along the plumb line from a point of interest to a reference surface known as the geoid, the vertical datum that approximates mean sea level. [1][2] Orthometric height is one of the scientific formalizations of a layman's "height above sea level", along with other types of heights in ...
Free-air gravity anomaly. In geophysics, the free-air gravity anomaly, often simply called the free-air anomaly, is the measured gravity anomaly after a free-air correction is applied to account for the elevation at which a measurement is made. It does so by adjusting these measurements of gravity to what would have been measured at a reference ...
Earth Gravitational Model. The Earth Gravitational Models (EGM) are a series of geopotential models of the Earth published by 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 formats: as the series of numerical coefficients to the spherical ...
Bouguer anomaly. In geodesy and geophysics, the Bouguer anomaly (named after Pierre Bouguer) is a gravity anomaly, corrected for the height at which it is measured and the attraction of terrain. [1] The height correction alone gives a free-air gravity anomaly. Bouguer anomaly map of the state of New Jersey (USGS)
The definition of NAVD 88 uses the Helmert orthometric height, which calculates the location of the geoid (which approximates MSL) from modeled local gravity. [citation needed] The NAVD 88 model is based on then-available measurements, and remains fixed despite later improved geoid models. [citation needed]
Geopotential is the gravitational potential energy per unit mass at elevation : where is the acceleration due to gravity, is latitude, and is the geometric elevation. [1] Geopotential height may be obtained from normalizing geopotential by the acceleration of gravity: where = 9.80665 m/s 2, the standard gravity at mean sea level. [4]
The separation between the geoid and the reference ellipsoid is called the undulation of the geoid, symbol . The geoid, or mathematical mean sea surface, is defined not only on the seas, but also under land; it is the equilibrium water surface that would result, would sea water be allowed to move freely (e.g., through tunnels) under the land.
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