GRS 80

The Geodetic Reference System 1980 (GRS 80) is a geodetic reference system consisting of a global reference ellipsoid and a gravity field model.Geodesy 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.The reference ellipsoid is usually defined by its semi-major axis (equatorialradius) a {displaystyle a}   and either its semi-minor axis (polar radius) b {displaystyle b}  , aspect ratio ( b / a ) {displaystyle (b/a)}   or flattening f {displaystyle f}  , but GRS80 is an exception: For a complete definition, four independent constants are required. GRS80 chooses as these a {displaystyle a}  , G M {displaystyle GM}  , J 2 {displaystyle J_{2}}   and ω {displaystyle omega }  , making the geometrical constant f {displaystyle f}   a derived quantity.