1-17. The World Geodetic System. Because local or even preferred systems fail to provide
intercontinental geodetic information, a unified world system has become essential. To establish a
world system, all of the available observed data must be considered. This will determine the absolute
reference system which best fits the entire earth. Once such a system is established, computed geodetic
coordinates will be comparable worldwide. To develop such a system for our military purposes, all
branches of the military have actively participated in a program to develop a Department of Defense
(DOD) WGS.
a. Because of its intercontinental-range missile capability, the United State Air Force (USAF) has
played an active role in the development of a WGS. Due to the wide scope of the project, USAF
participation has extended over a 20-year period in order to obtain sufficient data upon which to base a
world system.
b. Active participation by various USAF agencies has permitted the gathering of extensive data
needed to formulate a unified WGS. An important phase of this preparatory work is the HIRAN
trilateration net which spans the North Atlantic Ocean from the eastern coast of Canada to Norway.
This tie permits connection of the North American and European Datums. However, since both of these
datums are based on ellipsoids which provide a good fit only in the areas of their origins, connecting the
two datums through the HIRAN tie overextends the usefulness of either ellipsoid. We have seen how
large, systematic geoid separations can occur when a geodetic datum is extended far from its origin.
Nevertheless, ties made with HIRAN will provide effective checks when the two major datums are
oriented to a common absolute ellipsoid. Another significant example of HIRAN trilateration is the
West Indies loop.
c. A second major area in which preparatory work has been done in anticipation of a WGS is in the
collection and analysis of gravity observations. Through an extensive program, several thousand gravity
reference stations have been interconnected throughout the world. The connection of these base stations
by airplane and gravimeter has permitted the reduction of numerous observations to a common usable
system and, in effect, has established a world gravity system. This has permitted application to geodetic
problems of data collected for oil prospecting and other geophysical purposes.
(1) The collection of extensive gravity data permitted preparation of mean gravity anomalies for
1 x 1 and 5 x 5 degree geographical sectors that are needed for further gravimetric computations. It also
permitted preparing the gravimetric geoid used in the orientation of the ellipsoid for the NIMA WGS.
Much of the work in the collection and the analysis of gravity data was done by contracts with civilian
universities.
(2) An additional phase of gravity work includes computing gravity anomalies for places in
which there are no observations. As stated previously, gravimetric computations require knowledge of
gravity anomalies over the entire earth. Some regions of the earth are completely void of observation
points, and the best substitute must be found. A theoretical anomaly can be found by considering the
topography near the point and making certain applications of our knowledge of the earth's crust.
Although the computations for each point are tedious and involve consideration of the
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