- GRS 80
GRS 80, or Geodetic Reference System 1980, is a
geodetic reference systemconsisting of a global reference ellipsoidand a gravity field model.
Geodesy, also called geodetics, is the scientific discipline that deals with the measurement and representation of the earth, its gravitational field and geodynamic phenomena ( polar motion, earth tides, and crustal motion) in three-dimensional, time-varying space.
geoidis essentially the figure of the Earth abstracted from its topographic features. It is an idealized equilibrium surface of sea water, the mean sea level surface in the absence of currents, air pressure variations etc. and continued under the continental masses. The geoid, unlike the ellipsoid, is irregular and too complicated to serve as the computational surface on which to solve geometrical problems like point positioning. The geometrical separation between it and the reference ellipsoid is called the geoidal . It varies globally between ±110 m.
reference ellipsoid, customarily chosen to be the same size (volume) as the geoid, is described by its semi-major axis (equatorialradius) "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 (GRS80) posited a 6,378,137 m semi-major axis and a 1:298.257 flattening. This system was adopted at the XVII General Assembly of the International Union of Geodesy and Geophysics (
IUGG). 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(equatorialradius) and either its semi-minor axis(polar radius) , aspect ratioor flattening. For GRS80, these are:
geometrical constants :
:Semi-major axis = Equatorial Radius 6,378,137.00000 m;
:Semi-minor axis = Polar Radius 6,356,752.31414 m;
:Aspect ratio 0.996647189318775;
:Reciprocal of flattening 298.257222101;
; 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
: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 geometrical 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).
*Additional derived physical constants and geodetic formulas are found in the following reference: Geodetic Reference System 1980, Bulletin Géodésique, Vol 54:3, 1980.
* [http://www.gfy.ku.dk/~iag/HB2000/part4/grs80_corr.htm GRS 80 Specification]
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