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World Geodetic System: WGS 84

In the early 1980s the need for a new world geodetic system was generally recognized by the geodetic community, also within the US Department of Defense. WGS 72 no longer provided sufficient data, information, geographic coverage, or product accuracy for all then current and anticipated applications. The means for producing a new WGS were available in the form of improved data, increased data coverage, new data types and improved techniques. GRS 80 parameters together with available Doppler, satellite laser ranging and Very Long Baseline Interferometry (VLBI) observations constituted significant new information. An outstanding new source of data had become available from satellite radar altimetry. Also available was an advanced least squares method called collocation which allowed for a consistent combination solution from different types of measurements all relative to the Earth's gravity field, i.e. geoid, gravity anomalies, deflections, dynamic Doppler, etc.

The new World Geodetic System was called WGS 84. It is currently the reference system being used by the Global Positioning System. It is geocentric and globally consistent within ±1 m. Current geodetic realizations of the geocentric reference system family International Terrestrial Reference System (ITRS) maintained by the IERS are geocentric, and internally consistent, at the few-cm level, while still being metre-level consistent with WGS 84.

The WGS 84 originally used the GRS 80 reference ellipsoid, but has undergone some minor refinements in later editions since its initial publication. Most of these refinements are important for high-precision orbital calculations for satellites but have little practical effect on typical topographical uses. The following table lists the primary ellipsoid parameters.

Ellipsoid reference Semi-major axis a Semi-minor axis b Inverse flattening (1/f) GRS 80 6,378,137.0 m ≈ 6,356,752.314 140 m 298.257 222 101 WGS 84 6,378,137.0 m ≈ 6,356,752.314 245 m 298.257 223 563

The very small difference in the flattening thus results in a—very theoretical—difference of 105 µm in the semi polar axis. For most purposes, the differing polar axes can be merged to 6,356,752.3 m, with the inverse flattening rounded to 298.257.

WGS84 receiver indicating its reference median is 5.3 arcseconds east Longitudes on WGS 84 WGS 84 uses the IERS Reference Meridian as defined by the Bureau International de l'Heure, which was defined by compilation of star observations in different countries. The mean of this data caused a shift of about 100 metres east away from the Prime Meridian at Greenwich, UK. The longitude positions on WGS 84 agree with those on the older North American Datum 1927 at roughly 85° longitude west, in the east-central United States.

Updates and new standards
The latest major revision of WGS 84 is also referred to as "Earth Gravitational Model 1996" (EGM96), first published in 1996, with revisions as recent as 2004. This model has the same reference ellipsoid as WGS 84, but has a higher-fidelity geoid (roughly 100 km resolution versus 200 km for the original WGS 84).

Many of the original authors of WGS 84 contributed to a new higher fidelity model, called EGM2008. This new model will have a geoid with a resolution approaching 10 km, requiring over 4.6 million terms in the spherical expansion (versus 130,317 in EGM96 and 32,757 in WGS 84).

 
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