| Grs80 |
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GRS 80, or '''Geodetic Reference System 1980''', is a Geodetic Reference System consisting of a global Reference Ellipsoid and a Gravity Field Model . CONTEXT Geodesy , also called geodetics, is the scientific discipline that deals with the measurement and representation of the Earth , its Gravitation al field and geodynamic phenomena ( Polar Motion , earth Tide s, and crustal motion) in three-dimensional, time-varying space. The . 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''. The quantity ''f'' = (''a''−''b'')/''a'', where ''b'' is the semi-minor axis (polar radius), is a purely geometrical one. The mechanical ellipticity of the earth (dynamical flattening, symbol ''J''2) is determined to high precision by observation of satellite orbit perturbations. Its relationship with the geometric flattening is indirect. The relationship depends on the internal density distribution, or, in simplest terms, the degree of central concentration of mass. The 1980 Geodetic Reference System (). It is essentially the basis for geodetic positioning by the Global Positioning System and is thus also in extremely widespread use outside the geodetic community. The numerous other systems which have been used by diverse countries for their maps and charts are gradually dropping out of use as more and more countries move to global, geocentric reference systems using the GRS80 reference ellipsoid. DEFINING FEATURES OF GRS 80 The reference ellipsoid is defined by its Semi-major Axis (equatorial radius) and either its Semi-minor Axis (polar radius) , Aspect Ratio or Flattening . For GRS80, these are: ; Defining Geometrical Constant s : :Semi-major axis = Equatorial Radius 6,378,137.00000 m; :Semi-minor axis = Polar Radius 6,356,752.31414 m; :Aspect ratio 0.996647189318775; :Flattening 0.003352810681225; :Reciprocal of flattening 298.2572220972; ; Derived geometrical constants : := 6,372,797.5559 m; := 6,371,007.1810 m; :Radius of a sphere of the same volume 6,371,000.7900 m; :Linear eccentricity 521,854.0097 m; :Polar radius of curvature 6,399,593.6259 m; :Meridian quadrant = 10,001,965.7293 m; ; Defining Physical Constants : :Geocentric gravitational constant, including mass of the atmosphere 3986005·108 m3/s2; :Dynamical form factor = 108263· 10-8; :Angular velocity of rotation = 7292115·10-11 s-1; For a complete definition, ''four'' independent constants are required. GRS80 chooses as these , , and , making the geometical constant a derived quantity. The GRS80 reference system is used by the Global Positioning System , in a realization called WGS 84 (World Geodetic System 1984). REFERENCES
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