Establishment of Geodetic Coordinate Systems - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 337

Fig. 7.5 Geodetic origin
data and ellipsoid
orientation
origin. The geodetic coordinates of the origin L 0 , B 0 , H 0 and its geodetic azimuth A 0
with reference to a certain direction are called the geodetic origin data, which are
the coordinate datum for classical geodetic survey.
The ellipsoid orientation is closely related to the determination of the geodetic
origin data. To position and orient the ellipsoid is to determine the geodetic origin
data, and once the geodetic origin data are determined, the positioning and orien-
tation of the ellipsoid will be completed. As shown in Fig. 7.5 , L 0 , B 0 of the geodetic
origin P define the normal to the ellipsoid passing through this point, but the
ellipsoid can still rotate and translate about the surface normal, which will not be
completely fixed unless H 0 and A 0 are determined.
Mathematically, however, we determine the position and orientation; any set of
L 0 , B 0 , H 0 , and A 0 will enable us to define the relationship between the ellipsoid and
the geoid, yet such a relationship not appropriate. The reference ellipsoid is a
mathematical figure of the geoid and we should attempt to make it better approx-
imate the regional geoid. This is the only way that the observed elements reduced to
the ellipsoid can be practically significant. Meanwhile, the deflection of the vertical
and the initial geodetic azimuth will also be conveniently solved. Hence, the
positioning and orientation of the ellipsoid are required to satisfy the following
conditions:
1. The minor axis of the ellipsoid is parallel to the Earth's rotation axis
2. The planes of the initial geodetic and astronomical meridians are parallel to each
other
3. The ellipsoid surface best fits the geoid in an area of interest