The coordinate origin of WGS 84 is meant to be located at the Earth's center of mass; the error is believed to be less than 2 cm.[2]
The WGS 84 meridian of zero longitude is the IERS Reference Meridian,[3] 5.31 arc seconds east of the Greenwich Prime Meridian, or 102.5 metres (336.3 feet) at the latitude of the Royal Observatory.[4][5]
The WGS 84 datum surface is an oblate spheroid (ellipsoid) with major (transverse) radius a = 6,378,137 m at the equator and flattening f = 1/298.257223563.[6] The polar semi-minor (conjugate) radius b then equals a times (1 - f), or 6,356,752.3142 m.[7]
Presently WGS 84 uses the 1996 Earth Gravitational Model (EGM96) geoid, revised in 2004. This geoid defines the nominal sea level surface by means of a spherical harmonics series of degree 360 (which provides about 100 km horizontal resolution).[8] The deviations of the EGM96 geoid from the WGS 84 reference ellipsoid range from about -105 m to about +85 m.[9] EGM96 differs from the original WGS 84 geoid, referred to as EGM84.