What is WGS-84?
The World Geodetic System provides a standard co-ordinate frame for the Earth, a standard spheroidal reference surface (or ellipsoid) for raw altitude data, and a gravitational equipotential surface (or geoid) that defines the nominal sea level. WGS-84 is the most recent version of the system, which was originated in 1984 and revised in 2004. Earlier schemes included WGS-72, WGS-66 and WGS-60. WGS-84 is the reference coordinate system used by GPS.
The origin of the WGS-84 co-ordinate system is centred close to the Earth's centre of mass. Indeed, the error is reckoned to be less than 20mm. The meridian of zero longitude is the IERS Reference Meridian, which lies 5.31 arc seconds east of the Greenwich Prime Meridian.
The WGS-84 datum surface is an oblate spheroid, with a major radius of 6,378,137m at the equator, and minor radius of 6,356,752m at the poles.
WGS-84 currently uses the 1996 Earth Gravitational Model (EGM96) geoid, which was revised in 2004. This defines the nominal sea level surface by means of a spherical harmonics series of degree 360 (which provides about 100km horizontal resolution).
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