Establishment of Geodetic Coordinate Systems - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 344

7.3.3 Significance of the Classical Method of Ellipsoid
Orientation in Understanding the Principle
of Establishing a Modern Geodetic Coordinate System
The principle of fixing the coordinate system to the Earth passing through surface
points P and M in Fig. 7.5 is based on the precondition that the Earth is assumed to
be rigid. The Earth is actually a non-rigid body, in fact it is a complex viscoelastic
body. As a result, the surface points are subject to constant changes, which include
both regular changes like the solid Earth tide and irregular changes like various
unpredictable deformations. Hence, the coordinate systems cannot be accurately
determined only from points P and M. Such indetermination can be improved by
increasing the number of surface points used to define the coordinate system and by
long-term repeated observations. The coordinates of these surface points should
certainly not contradict each other; for example, the distance between points is
subject to objective constraints. Then, we can use relative measuring techniques
like GPS or VLBI to determine the distance between points.
It is thus clear that the chosen coordinate system fixed to the Earth is realized by
determining the coordinates of a set of surface points (datum points). Alternatively,
we might say that a self-consistent set of station coordinates contains
(or determines) the coordinate system. These points are the VLBI, SLR, and GPS
points. The fundamental principle of establishing a modern geodetic coordinate
system is given above.
7.4 Conventional Terrestrial Reference System
7.4.1 The Geocentric Coordinate System and Its Application
A coordinate system with its origin at the center of mass of the Earth is called a
geocentric coordinate system. Likewise, a coordinate system with its origin at the
geometric center of the reference ellipsoid based on classical surveying techniques
is called a non-geocentric coordinate system (also referred to as the local coordinate
system). Identical with the local coordinate system, the geocentric coordinate
system can also be categorized into geocentric geodetic coordinate system and
geocentric geodetic Cartesian coordinate system. In modern geodesy, a geocentric
coordinate system is usually referred to as a terrestrial reference system (TRS). The
pointing direction of the coordinate axes of the TRS changes under the effect of the
Earth's polar motion, which will present obstacles in practical use. Consequently,
as early as 1967, the International Astronomical Union (IAU) and the International
Association of Geodesy (IAG) recommended that the 1900-1905 mean latitude of
the five International Latitude System stations be used as the datum point. The
position of the mean pole corresponds to the mean position of the rotation axis over