The US Department of Defense World Geodetic System 1972
After an extensive effort extending 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 nonDoD 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 nonDoD 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 (Figure 38). 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 BC4 camera system
(Figure 39). Data from the Smithsonian Astrophysical Observatory was also used
which included camera (Baker Nunn) and some laser ranging.
Doppler satellite ground stations providing data for WGS 72 development
Worldwide geometric satellite triangulation network, BC4 camerasThe 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
gravitygeophysical correlation techniques. Approximately 45 percent of the 410
mean free air gravity anomaly values were determined directly from observed
gravity data.
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 BC4, SECOR, Doppler and
BakerNunn systems. Also, eight geodimeter long line precise traverses were
included for the purpose of controlling the scale of the solution.
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.
The value for the semimajor axis (a) of the WGS 72 Ellipsoid is 6 378 135
meters. The adoption of an avalue 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 localtogeocentric 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 astrogeodetic datum
orientation stations were included in the various solutions. Based on these
results and other related studies accomplished by the Committee, an avalue of 6
378 135 meters and a flattening of 1/298.26 were adopted.
In the development of localto 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.
