A New 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 0.105 mm 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.
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