Efforts to supplement the various national
surveying systems began in the 19th century with
F.R. Helmert's book (
Mathematical and Physical Theories of Physical Geodesy).
Austria and
Germany founded the (Central Bureau of International
Geodesy), and a series of global
ellipsoids of the Earth were derived (e.g., Helmert 1906,
Hayford 1910 and 1924). A unified geodetic system for the whole world became essential in the 1950s for several reasons: • International
space science and the beginning of
astronautics. • The lack of inter-continental geodetic information. • The inability of the large
geodetic systems, such as European Datum (
ED50),
North American Datum (NAD), and
Tokyo Datum (TD), to provide a worldwide geo-data basis • Need for global maps for
navigation, aviation, and
geography. • Western
Cold War preparedness necessitated a standardised,
NATO-wide geospatial reference system, in accordance with the NATO
Standardisation Agreement WGS 60 In the late 1950s, the
United States Department of Defense, together with
scientists of other institutions and countries, began to develop the needed world system to which geodetic data could be referred and compatibility established between the coordinates of widely separated sites of interest. Efforts of the U.S. Army, Navy and Air Force were combined leading to the DoD World Geodetic System 1960 (WGS 60). The term
datum as used here refers to a smooth surface somewhat arbitrarily defined as zero elevation, consistent with a set of surveyor's measures of distances between various stations, and differences in elevation, all reduced to a grid of
latitudes,
longitudes, and
elevations. Heritage surveying methods found elevation differences from a local horizontal determined by the
spirit level,
plumb line, or an equivalent device that depends on the local gravity field (see
physical geodesy). As a result, the elevations in the data are referenced to the
geoid, a surface that is not readily found using
satellite geodesy. The latter observational method is more suitable for global mapping. Therefore, a motivation, and a substantial problem in the WGS and similar work is to patch together data that were not only made separately, for different regions, but to re-reference the elevations to an ellipsoid model rather than to the
geoid. In accomplishing WGS 60, a combination of available surface
gravity data,
astro-geodetic data and results from HIRAN and Canadian
SHORAN surveys were used to define a best-fitting
ellipsoid and an earth-centered orientation for each initially selected datum. (Every datum is relatively oriented with respect to different portions of the geoid by the astro-geodetic methods already described.) The sole contribution of
satellite data to the development of WGS 60 was a value for the
ellipsoid flattening which was obtained from the nodal motion of a satellite. Prior to WGS 60, the U.S. Army and
U.S. Air Force had each developed a world system by using different approaches to the gravimetric datum orientation method. To determine their gravimetric orientation parameters, the Air Force used the mean of the differences between the gravimetric and astro-geodetic
deflections and geoid heights (undulations) at specifically selected stations in the areas of the major datums. The Army performed an adjustment to minimize the difference between astro-geodetic and
gravimetric geoids. By matching the relative astro-geodetic geoids of the selected datums with an earth-centered gravimetric geoid, the selected datums were reduced to an earth-centered orientation. Since the Army and Air Force systems agreed remarkably well for the NAD, ED and TD areas, they were consolidated and became WGS 60.
WGS 66 Improvements to the global system included the Astrogeoid of
Irene Fischer and the astronautic Mercury datum. In January 1966, a World Geodetic System Committee composed of representatives from the United States Army, Navy and Air Force was charged with developing an improved WGS, needed to satisfy
mapping, charting and geodetic requirements. Additional surface
gravity observations, results from the extension of
triangulation and
trilateration networks, and large amounts of
Doppler and
optical satellite data had become available since the development of WGS 60. Using the additional data and improved techniques, WGS 66 was produced which served DoD needs for about five years after its implementation in 1967. The defining parameters of the WGS 66 Ellipsoid were the flattening ( determined from satellite data) and the semimajor axis ( determined from a combination of Doppler satellite and astro-geodetic data). A worldwide 5° × 5° mean free air
gravity anomaly field provided the basic data for producing the WGS 66 gravimetric geoid. Also, a geoid referenced to the WGS 66 Ellipsoid was derived from available astrogeodetic data to provide a detailed representation of limited land areas.
WGS 72 After an extensive effort over a period of approximately three years, the Department of Defense World Geodetic System 1972 was completed. Selected satellite, surface gravity and astrogeodetic data available through 1972 from both DoD and non-DoD sources were used in a Unified WGS Solution (a large scale
least squares adjustment). The results of the adjustment consisted of corrections to initial station coordinates and coefficients of the gravitational field. The largest collection of data ever used for WGS purposes was assembled, processed and applied in the development of WGS 72. Both optical and electronic satellite data were used. The electronic satellite data consisted, in part, of Doppler data provided by the U.S. Navy and cooperating non-DoD satellite tracking stations established in support of the Navy's Navigational Satellite System (NNSS). Doppler data was also available from the numerous sites established by GEOCEIVERS during 1971 and 1972. Doppler data was the primary data source for WGS 72 (see image). Additional electronic satellite data was provided by the SECOR (Sequential Collation of Range) Equatorial Network completed by the U.S. Army in 1970. Optical satellite data from the Worldwide Geometric Satellite Triangulation Program was provided by the BC-4 camera system (see image). Data from the
Smithsonian Astrophysical Observatory was also used which included camera (
Baker–Nunn) and some laser ranging. This resulted in a tiny difference of in the semi-minor axis. The following table compares the primary ellipsoid parameters. ==Definition==