World Geodetic System 84
From time to time, an Earth-centred reference system is adopted which is a best fit for the whole Earth and based on the latest information.
The reference system adopted in 1980, known as the Geodetic Reference System 1980 (GRS80) was used by the United States Defence Mapping Agency as the basis for the World Geodetic System 1984 (WGS84), which is currently used for the Global Positioning System (GPS) satellite navigation system.
The parameters of the WGS84 ellipsoid "... are identical to those for the GRS80 ellipsoid with one minor exception. The coefficient form used for the second degree zonal is that of the WGS84 Earth Gravitational Model rather than the notation J2 used with GRS80." (DMA, 1987).
The Geocentric Datum of Australia (GDA) - GDA Technical Manual provides a greater explanation of the relationship between WGS84 and GDA94.
The WGS84 ellipsoid parameters are:
|Semi-major axis (a):||6 378 137.00 metres|
|Inverse Flattening (1/f):||298.257223563|
Department of Defense, 1987. "Department of Defense World Geodetic System 1984 - Its Definition and Relationships with Local geodetic Coordinate Systems", DMA Technical Report 8350.2
World Geodetic System 1972 (WGS72)
The World Geodetic System 1972 (WGS72) was the third Earth-centred reference frame developed by the United States Defense Mapping Agency (DMA) to support its activities (previous versions were WGS60 and WGS66). It has since been superseded by WGS84, but until 27 January 1987, WGS72 was used with the GPS system and prior to 27 January 1989 it was used for the Transit Doppler navigation system broadcast ephemeris. DMA has published transformation parameters to change between WGS72 and WGS84.
The WGS72 ellipsoid parameters are:
|Semi-major axis (a):||6 378 135.0 metres|
|Inverse Flattening (1/f):||298.26|
Defense Mapping Agency 1987, Department of Defense World Geodetic System 1984, its definition and relationships with local geodetic systems.
Jenkins, RE and Leroy, CF 1974. Broadcast versus Precise Ephemeris - Apples and Oranges - Proceedings of the Second International Symposium on Satellite Doppler Positioning, Austin Texas.
Seppelin, TO 1974 The Department of Defense World Geodetic System 1972. International Symposium on Problems Relating to the Redefinition of the North American Geodetic Networks. Fredrickton, New Brunswick.