Definition

GRS 80, or Geodetic Reference System 1980, is a geodetic reference system consisting of a global reference ellipsoid and a gravity field model. In geodesy, a reference ellipsoid is a mathematically defined surface that approximates the true figure of the Earth or geoid. ...

The reference elllipsoid is defined by its semi-major axis (equatorial radius) a and either its semi-minor axis (polar radius) b, aspect ratio (b / a) or flattening f. For GRS80, these are: In geometry, the term semi-major axis (also semimajor axis) is used to describe the dimensions of ellipses and hyperbolas. ... In geometry, the semi-minor axis (also semiminor axis) applies to ellipses and hyperbolas. ... The aspect ratio of a two-dimensional shape is the ratio of its longest dimension to its shortest dimension. ... The flattening, ellipticity, or oblateness of an oblate spheroid is the relative difference between its equatorial radius a and its polar radius b: The flattening of the Earth is 1:298. ...

Defining geometrical constants 

Semi-major axis = Equatorial Radius = a = 6,378,137.00000 m;

Semi-minor axis = Polar Radius = b = 6,356,752.31414 m;

Aspect ratio = b / a = 0.996647189318775;

Flattening = f = 0.003352810681225;

Reciprocal of flattening = 1 / f = 298.2572220972;

Derived geometrical constants 

Quadratic mean radius= 6,372,797.5559 m;

Authalic mean radius= 6,371,007.1810 m;

Radius of a sphere of the same volume = (a²b)^{1 / 3} = 6,371,000.7900 m;

Linear eccentricity = (a² − b²)^.5 = 521,854.0097 m;

Polar radius of curvature = a² / b = 6,399,593.6259 m;

Meridian quadrant = 10,001,965.7293 m;

Defining physical constants 

Geocentric gravitational constant, including mass of the atmosphere GM = 3986005·10⁸ m³/s²;

Dynamical form factor J₂ = 108263· 10^-8;

Angular velocity of rotation ω = 7292115·10^-11 s^-1;

For a complete definition, four independent constants are required. GRS80 chooses as these a,GM,J₂ and ω, making the geometical constant f a derived quantity. Geometry (from the Greek words Ge = earth and metro = measure) is the branch of mathematics first introduced by Theaetetus dealing with spatial relationships. ... In mathematics and the mathematical sciences, a constant is a fixed, but possibly unspecified, value. ... The radius of the Earth is the distance from the Earths centre to its surface at mean sea level. ... The radius of the Earth is the distance from the Earths centre to its surface at mean sea level. ... Antonym of psychical. ... In mathematics and the mathematical sciences, a constant is a fixed, but possibly unspecified, value. ...

Additional derived physical constants and geodetic formulas are found in the following reference: Geodetic Reference System 1980, Bulletin Géodésique, Vol 54:3, 1980.

The GRS80 reference system is used by the Global Positioning System, in a realization called WGS 84 (World Geodetic System 1984). Over fifty GPS satellites such as this NAVSTAR have been launched since 1978. ... WGS 84 is the 1984 revision of the World Geodetic System. ...