1984 World Geodetic System

The most widely used reference model of the Earth; that is, world's standard global geodetic reference system, or datum. Originally derived for US military satellite navigation, it is now used worldwide for aeronautical navigation, global positioning systems, and consequently as a reference for geographic information systems.

Although localized datums tied to a specific point on the surface of the Earth are very accurate for individual countries, significant errors show up when such a system is extended to the entire globe. The United States and Canada were large enough that datum-introduced errors caused problems merely for land-based surveying and mapping. The 1927 North American Datum, or NAD 27, based at Meades Ranch, Kansas, cannot be used for mapping Northern Alaska, not to mention Ellesmere Island. The United States Geological Survey and the Canadian Geodetic Survey began the development of a new datum based in North Dakota, NAD 83.

A funny thing happened along the way, however: The world entered the Space Age. When you are attempting to calculate the orbits of satellites, using several datums in a piecewise fashion only complicates software and compounds errors. And so the development of NAD 83 was changed to tie it to the Earth's center of mass. USGS adopted the International Union of Geodesy and Geophysics's World Geodetic System 1980 for this purpose. During the early 1980's, USGS began revising their maps to show the offsets for NAD 83; differences of 100 to 300 meters at various points around the country.

Of course, the biggest users of global geographic references are the United States Armed Forces. All along, the Defense Mapping Agency had been developing its own datums for military spy satellite and aeronautical navigation.

The driving force behind WGS 84 was the Navy Navigation Satellite System, also known as TRANSIT, which came on line in 1987. WGS 84 got a big boost in 1989 when the International Civil Aviation Organization adopted it for civilian air navigation. When Global Positioning Systems were made available for civilian use in the 1990's, WGS 84 became something everyone should know about.

WGS 84 is a rotation of GRS 80 so that the prime meridian lines up with the one defined by the Bureau International de l'Heure (or BIH). The Earth is projected onto an ellipsoid, with the following characteristics.

semimajor (equatorial) axis        a = 6,378,137.0 m
flattening                         f = 0.003352810665   (1/298.257223563)
angular velocity                   ω = 7.292115e-5 rad/s
geocentric gravitational constant  GM = 398600.5 km3/s2

Notice how physical constants1 such as mass and angular velocity come into the picture; these are necessary for calculating satellite orbits. The following measurements can be derived from the above:

semiminor (polar) axis             b = 6,356,752.314 m
eccentricity                       e = 0.08181919 
rotational period                    = 86164.102 s
mass of Earth                      M = 5.9737e24 kg = 5.9737e21 metric tons

A reference frame for the Earth (or "realization") is imposed on this ellipsoid by calculating GPS signals relative to five Air Force GPS control stations around the world (Hawaii, Cheyenne Mountain, Ascension Island, Diego Garcia, and Kwajalein).

Accompanying the reference frame is a worldwide matrix of differences between the ellipsoid and a gravitationally equipotential "mean sea level" shape, or geoid. Added on top of that is a coverage of gravitational anomalies, fluctuations in the Earth's gravitational field due to things like mountains or rocks of varying densities. Realizations of WGS 84, as well as the geoid, have undergone continual refinement since 1987; NIMA's latest is current to 1999.

Using these parameters, a GPS receiver can calculate its position on the Earth with one meter of horizontal accuracy when only one satellite has been acquired. GPS readings accurate to a centimeter come from averaging values from several satellites, or coordinating signals from local aids to navigation (differential GPS).

Being based entirely on GPS readings, WGS 84 isn't accurate enough for some uses. Scientific geodesy and NAD 83 now use a somewhat more accurate datum than WGS 84, the International Terrestrial Reference System (ITRS), which is better at taking plate tectonics into account. However, its coordinates are within a centimeter of WGS 84 globally.

1Another "primary parameter" of WGS 84 is the "normalized 2nd degree zonal harmonic coefficient of the gravitational potential", a dimensionless number (-0.00048416685). I have no idea what this mouthful means, but it probably has something to do with the effects of general relativity on GPS satellite orbits.

Richard A. Snay and Tomas Soler, "Modern Terrestrial Reference Systems", Professional Surveyor, January thru April 2000, available in four parts at http://www.ngs.noaa.gov/CORS/Articles/Reference-Systems-Part-1.pdf (Similarly, -part-2.pdf, -part-3.pdf, -part-4.pdf)

John P. Snyder: Map Projections: A Working Manual. U.S. Geological Survey Progessional Paper 1395, U.S. Government Printing Office, Washington, 1987

WGS 84 Implementation Manual, 1998, prepared by EUROCONTROL (European Organization for the Safety of Air Navigation, Brussels) and IfEN (Institute for Geodesy and Navigation, Munich), available at http://www.wgs84.com/files/wgsman84.pdf

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