Introduction - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 2

To our knowledge, geodesy is both a foundational and an applied discipline. As
an applied discipline, geodesy, in the subject catalogue of some countries like
China, is a subbranch of geomatics and surveying and mapping. Geomatics and
surveying and mapping mainly study all forms of the Earth's surface; therefore the
shape and size of the Earth and its gravity field are studied and measured, and a
unified coordinate system should be established to show the exact geometric
positions of an arbitrary point on the Earth's surface. Hence, geodetic measure-
ments usually need to be carried out before topographic mapping. On the other
hand, as a foundational discipline, geodesy, in the subject catalogue of some
countries like China, is a subbranch of geophysics. Geophysics is primarily
concerned with the Earth's movement, state, components, acting force, and all
kinds of physical processes. Hence, geodesy provides instant, dynamic, and quan-
titative spatial geometric and physical information with high accuracy and resolu-
tion, which serves as an important means of studying geodynamic phenomena such
as the Earth's rotation, movement of the Earth's crust, and changes of sea surface,
and is used for prediction of geological disasters.
1.1.2 Classifications of Geodesy
According to the scope of the geospace studied, geodesy can be classified into
ellipsoidal geodesy (i.e., theoretical geodesy, higher surveying), geodetic control
survey, marine geodesy, and engineering geodesy (i.e., plane surveying). Ellipsoi-
dal geodesy studies the body of the Earth as a whole, determines the shape of the
Earth and its external gravity field, and establishes the geodetic reference system.
Geodetic control survey measures the coordinates and heights of a sufficient
number of surface points within one or several countries in an appropriately chosen
reference system and establishes a unified national geodetic network to meet the
needs of topographic mapping and engineering construction. Marine geodesy
establishes a geodetic control network on the Earth's surface covered by oceans
to realize positioning on the sea surface and underwater and to measure the marine
gravity field, sea surface topography, and marine geoid. Engineering geodesy
determines the details on the Earth's surface regionally in a small area and usually
refers to the horizontal plane for measurement. Ellipsoidal geodesy, geodetic
control survey, marine geodesy, and engineering geodesy are closely related to
one another. National geodetic control survey and marine geodesy need the geo-
detic constants and reference datums determined by global geodetic surveying in
order to reduce the observational results taking into consideration the effect of the
Earth's curvature and the gravity field. The results obtained from the national
geodetic control survey and marine geodesy provide ellipsoidal geodesy with
information of geometric and physical measurements of the Earth's surface. Engi-
neering geodesy has to be connected with the national geodetic control network to
bring its results into the national unified coordinate system.