Introduction - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 17

1. Separated one-dimensional (vertical) and two-dimensional (horizontal) geodetic
measurements have been developed into three- and four-dimensional measure-
ments, which include time variables.
2. Measurement of the geometric and gravity field elements on the Earth's surface
under the hypothesis of a static and rigid Earth has been developed into the
observation of dynamic variations of the nonrigid (elastic and rheological) Earth.
3. Regional (relative) geodetic measurement in the local reference coordinate
system has been developed into global (absolute) geodetic measurement in the
unified geocentric coordinate system.
4. Measurement accuracy has been increased by two to three orders of magnitude.
These transformations have greatly expanded the research area of geodesy and
have formed modern geodesy as opposed to classical geodesy.
Developing Towards Basic Research Areas in the Geosciences
The potential of development of modern geodetic techniques indicates the possi-
bility of better monitoring of the Earth's movement and changes in its shape and
potential field with sufficient accuracy on any spatial-temporal scale. The changes
in the Earth's geometric and physical state are the result of its dynamic process
under the effect of both internal and external forces. Geodesy serves not only to
monitor and depict all kinds of geodynamic phenomena but also, more importantly,
to explain their mechanism of occurrence and to predict their evolutionary pro-
cesses. This is geodetic inversion, which includes the geophysical inversion of the
crustal movement and changes in the Earth's rotation and gravity field. In other
words, it means to infer, in reverse, the Earth's internal structural state, the source of
forces, and the parameters of dynamic processes from the geodetic observational
data of temporal variations. This intersectant research area between geodesy and its
related geosciences has formed a new branch, dynamic geodesy. This is currently
the most active interdisciplinary subfield of geodesy, whose development relies on
the development of space geodesy and physical geodesy and is also closely related
to the development of relevant geosciences. Dynamic geodesy is relatively inde-
pendent, and its theoretical system and methods are still under establishment.
Modern geodesy is developing mainly towards and into the geosciences. Its
basic tasks are:
1. To establish and maintain the inertial and terrestrial reference systems with high
accuracy; to build and maintain regional and global 3-D geodetic networks,
including the submarine geodetic network; to monitor in the long term the time-
varying changes of these networks on a certain time scale; and to provide
geodetic positioning and research of geodynamic phenomena with a highly
precise terrestrial reference frame and a network of surface reference points.
2. To monitor and explain various kinds of geodynamic phenomena, including
crustal movement, the Earth's rotational changes, the Earth's tide, sea surface
topography, and the variations in sea level.