are unified. An important feature of this method in its dependence on gravimetric undulations based on
observed gravity anomalies. The anomalies, in turn, depend on the gravity formula used. For the NIMA
program, the international gravity formula and a flattening of 1/297 are used. Before these geoid
undulations are used, corrections will be required for any flattening change adopted.
b. NIMA is participating in other important programs contributing to developing a WGS. These
programs, including celestial triangulation and star occultations, have been used primarily to connect
isolated islands to a world datum.
1-19. The Department of Defense World Datum. It is very probable that the world geodetic systems
determined by NIMA and the USAF will not agree in every respect. While the fundamental principles
involved in each approach are in harmony, the use of different groups of observed data is expected to
cause minor differences in the final results. Since a unified world system is desired, the two systems
obtained will be compared, and the major differences will be investigated by the DOD. However, the
task does not end there. New information will be gathered and evaluated to permit further refinement of
the world system. New geodetic methods, extended arc measurements, extended gravity surveys with
new measuring methods, and data obtained from earth satellites will all contribute to the determination
of a single world reference system that provides an increasingly close approximation to the true figure of
the earth.
1-41
EN0593
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