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The ellipsoid WGS-84, widely used for mapping and satellite navigation has f close to 1/300 ... and ellipsoidal height h (also known as geodetic height [8]).
The longitude positions on WGS 84 agree with those on the older North American Datum 1927 at roughly 85° longitude west, in the east-central United States. The WGS 84 datum surface is an oblate spheroid with equatorial radius a = 6 378 137 m at the equator and flattening f = 1 ⁄ 298.257 223 563.
Ellipsoidal height (or ellipsoidal altitude), also known as geodetic height (or geodetic altitude), is the distance between the point of interest and the ellipsoid surface, evaluated along the ellipsoidal normal vector; it is defined as a signed distance such that points inside the ellipsoid have negative height.
An approximate definition of sea level is the datum WGS 84, an ellipsoid, whereas a more accurate definition is Earth Gravitational Model 2008 (EGM2008), using at least 2,159 spherical harmonics. Other datums are defined for other areas or at other times; ED50 was defined in 1950 over Europe and differs from WGS 84 by a few hundred meters ...
Up to the 1960s, formulas based on the Hayford ellipsoid (1924) and of the famous German geodesist Helmert (1906) were often used. [citation needed] The difference between the semi-major axis (equatorial radius) of the Hayford ellipsoid and that of the modern WGS84 ellipsoid is 251 m; for Helmert's ellipsoid it is only 63 m.
GeographicLib provides a utility GeoidEval (with source code) to evaluate the geoid height for the EGM84, EGM96, and EGM2008 Earth gravity models. Here is an online version of GeoidEval . The Tracker Component Library from the United States Naval Research Laboratory is a free Matlab library with a number of gravitational synthesis routines.
The geometrical separation between it and the reference ellipsoid is called the geoidal undulation, or more usually the geoid-ellipsoid separation, N. It varies globally between ±110 m. A reference ellipsoid, customarily chosen to be the same size (volume) as the geoid, is described by its semi-major axis (equatorial radius) a and flattening f.
Ellipsoid Horizontal Datum CS Type Projection Origin Axes Unit of Measure 4326: GCS WGS 84: GRS 80: WGS 84: ellipsoidal (lat, lon) N/A: equator/prime meridian: equator, prime meridian: degree of arc 26717: UTM Zone 17N NAD 27: Clarke 1866: NAD 27: cartesian (x,y) Transverse Mercator: central meridian 81°W, scaled 0.9996: 500 km west of (81°W ...