Figure 1-18. Geoid Profile of the 98th Meridian
(f) Astro-geodetic datum orientation is obtained from the deflections of the vertical at a
number of selected Laplace stations (Figure 1-19). In this orientation method, the geoid and the
ellipsoid are oriented so that the sum of the squares of the deflections of the vertical at the Laplace
stations is made as small as possible. One of the Laplace stations in the adjustment is arbitrarily selected
as the origin. Consequently, instead of having a zero deflection as with the single-point orientation,
there is a deflection of the vertical at the origin point. Through a similar process, the geoid separation at
the starting point can be found. This procedure reorients the ellipsoid so that it provides the best average
fit at all the Laplace stations. The use of geodetic data produced by this method is still limited to
relatively small areas since the deflections of the vertical still remain relative. They are not comparable
with similar deflections derived from another adjustment. As in the single astronomic method, the
datum is not earth-centered and the rotational axis of the ellipsoid does not coincide with the axis of the
earth.
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EN0593
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