Figure 1-25. Using the Same Reference Ellipsoid for all Datums
f. A WGS will be established permitting the computation of distances and directions for use with
intercontinental-range missiles. The effect of such a system will be more far-reaching than its
application to intercontinental problems. Since the world system preserves the positional relationship of
points within a local or preferred datum, the unified world system will also meet the requirements for
geodetic information for short- or medium-range missiles. In this way, the air-to-ground missiles, the
ground-based intermediate-range missiles, the ocean-based intermediate-range missiles, and the
intercontinental-range missiles can all use the single WGS.
1-18. The National Imagery and Mapping Agency World System. NIMA is working to formulate a
world reference system. The approach used by NIMA is different from the approach of the USAF, but it
was vital that the fundamental concepts of methods were consistent.
a. The basic NIMA approach is to use the numerous arc measurements that span North and South
America, Europe, and Africa. These arcs are used to determine the ellipsoid that provides the best fit for
all the areas. In order to properly orient this ellipsoid with reference to the earth's axis of rotation and
the center of the earth, a combined astro-geodetic deflection and gravimetric method will be used. By
using numerous deflections in the western and eastern hemispheres, astro-geodetic undulations will be
computed. These undulations will then be compared with absolute gravimetric undulations, and an
adjustment will be made so that the difference between the astro-geodetic and gravimetric undulations is
at a minimum. The end result of this method of orientation will be an absolute reference system in
which all geodetic datums
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