(c) The significance of a gravity-oriented datum is the absolute nature of the coordinates
obtained (See Figure 1-21). If the astronomic observations and triangulation are correct, the positions
derived by this method will be absolute positions with respect to the ellipsoid chosen. Assuming that we
have an ellipsoid that is a good fit to the geoid, an absolute position is given by the direction of the
normal to the ellipsoid whose axis coincides with the earth's axis of rotation.
Figure 1-21. Astro-Gravimetric Datum Orientation
(d) Because the gravimetrically-oriented ellipsoid is earth-centered and aligned with the
earth's axis of rotation, various gravimetrically-adjusted datums will all be earth-centered and have the
minor axis of the ellipsoid coinciding with the rotational axis of the earth. This method of orientation
will be especially significant when discussing the unification of various geodetic datums to a single
worldwide system.
EN0593
1-32
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