entire earth, the theoretical anomaly is better than a blank observation square on a gravity anomaly map.
Filling in gravity maps with theoretical anomalies is not a substitute for actual observations, and data
continues to be gathered from both land and ocean gravity surveys.
d. The reference ellipsoid used in a WGS must be based on all available information. This includes
considering the data reduced from arc measurements in the eastern and western hemispheres as well as
the connecting tie from the HIRAN North Atlantic arc. These arcs provide the basis for determining the
ellipsoid's size. Recently, artificial earth satellites have provided a good method for determining the
shape and flattening of the earth. The flattening of the earth can be obtained from a long-lived satellite
placed in a high orbit. For example, the 1958 Vanguard satellite was tracked several thousand times and
indicated that a flattening of 1/298 is probable. This value was suggested before from the arc
measurements of Krassowsky. Krassowsky used the arc measurements as a datum element for the 1942
Russian datum. Earth satellites placed into orbit for the specific purpose of contributing geodetic data
help to confirm or revise this value of the earth's flattening.
e. Even with the best values obtainable for the size and the shape of the ellipsoid, the reference
ellipsoid must still be oriented to an absolute system. As mentioned earlier in this lesson, this can be
done by gravimetrically orienting the various preferred datum origins, using the same reference ellipsoid
for all datums (Figure 1-25). While we can use gravity anomalies to compute the deflection of the
vertical and geoid undulation for each datum, a new problem presents itself. The gravity anomalies that
we have available today are defined in terms of the international ellipsoid, even though the dimensions
of the WGS ellipsoid are different. As a result, the gravity formula must be revised to correspond to the
new ellipsoid and must be corrected in all the gravity anomalies. With the gravity anomalies consistent
with the reference ellipsoid, the gravimetric deflectors and undulations can be computed; and the major
world datums can be connected in absolute orientation on a common ellipsoid. Using this procedure, the
preferred datums are joined in the WGS. All that will remain to define a WGS will be the selection of
an initial point for the datum, and the selection will be quite arbitrary.
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EN0593
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