Editing World Geodetic System (section)

==={{anchor|WGS72}} WGS&nbsp;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.<ref name=":0">{{Cite book |url=https://www.ngs.noaa.gov/PUBS_LIB/Geodesy4Layman/toc.htm |title=Geodesy for the Layman |publisher=United States Air Force |year=1984 |language=en |chapter=THE WORLD GEODETIC SYSTEM |chapter-url=https://www.ngs.noaa.gov/PUBS_LIB/Geodesy4Layman/TR80003E.HTM#ZZ11}}</ref>

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 ([[Schmidt camera#Baker–Nunn|Baker–Nunn]]) and some laser ranging.<ref name=":0" />

[[Image:DOPPLER SATELLITE GROUND STATIONS PROVIDING DATA FOR WGS 72 DEVELOPMENT.GIF|thumb|upright=1.5|Doppler satellite ground stations providing data for WGS 72 development]]

[[Image:WORLDWIDE GEOMETRIC SATELLITE TRIANGULATION NETWORK, BC-4 CAMERAS.GIF|thumb|upright=1.5|Worldwide geometric satellite triangulation network, BC-4 cameras]]

The surface gravity field used in the Unified WGS Solution consisted of a set of 410 10° × 10° equal area mean free air gravity anomalies determined solely from terrestrial data. This gravity field includes mean anomaly values compiled directly from observed gravity data wherever the latter was available in sufficient quantity. The value for areas of sparse or no observational data were developed from geophysically compatible gravity approximations using gravity-geophysical correlation techniques. Approximately 45 percent of the 410 mean free air gravity anomaly values were determined directly from observed gravity data.<ref name=":0" />

The astrogeodetic data in its basic form consists of deflection of the vertical components referred to the various national geodetic datums. These deflection values were integrated into astrogeodetic geoid charts referred to these national datums. The geoid heights contributed to the Unified WGS Solution by providing additional and more detailed data for land areas. Conventional ground survey data was included in the solution to enforce a consistent adjustment of the coordinates of neighboring observation sites of the BC-4, SECOR, Doppler and Baker–Nunn systems. Also, eight [[geodimeter]] long line precise traverses were included for the purpose of controlling the scale of the solution.<ref name=":0" />

The Unified WGS Solution, as stated above, was a solution for geodetic positions and associated parameters of the gravitational field based on an optimum combination of available data. The WGS 72 ellipsoid parameters, datum shifts and other associated constants were derived separately. For the unified solution, a normal equation matrix was formed based on each of the mentioned data sets. Then, the individual normal equation matrices were combined and the resultant matrix solved to obtain the positions and the parameters.<ref name=":0" />

The value for the semimajor axis ({{mvar|a}}) of the WGS 72 Ellipsoid is {{val|6378135|u=meters}}. The adoption of an {{mvar|a}}-value 10 meters smaller than that for the WGS 66 Ellipsoid was based on several calculations and indicators including a combination of satellite and surface gravity data for position and gravitational field determinations. Sets of satellite derived station coordinates and gravimetric deflection of the vertical and geoid height data were used to determine local-to-geocentric datum shifts, datum rotation parameters, a datum scale parameter and a value for the semimajor axis of the WGS Ellipsoid. Eight solutions were made with the various sets of input data, both from an investigative point of view and also because of the limited number of unknowns which could be solved for in any individual solution due to computer limitations. Selected Doppler satellite tracking and astro-geodetic datum orientation stations were included in the various solutions. Based on these results and other related studies accomplished by the committee, an {{mvar|a}}-value of {{val|6378135|u=meters}} and a flattening of 1/298.26 were adopted.<ref name=":0" />

In the development of local-to WGS 72 datum shifts, results from different geodetic disciplines were investigated, analyzed and compared. Those shifts adopted were based primarily on a large number of Doppler TRANET and GEOCEIVER station coordinates which were available worldwide. These coordinates had been determined using the Doppler point positioning method.<ref name=":0" />