Geodetic Datum and Geodetic Control Networks - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 69

Chapter 3
Geodetic Datum and Geodetic Control
Networks
To measure terrain, surface features, position coordinates, heights, and gravity
values at points on the Earth's surface, there need to be corresponding reference
points or surfaces (also known as datum points or surfaces), namely geodetic
datums, to which surveying and mapping results are referred. Geodetic datums
consist chiefly of coordinate datums (including classical horizontal datums and
three-dimensional coordinate datums), vertical datums, sounding datums, as well as
gravity datums. Geodetic datums provide initial data for all kinds of surveying and
mapping work and serve as the foundation for determining the geometric shape and
spatial-temporal distribution of geospatial information. Again, it is geodetic
datums that are referred to when the spatial positions of geographical features in
the real world are expressed in the data space. The missions of constructing
geodetic datums include determining and defining the coordinate system, height
system, and gravity reference system, and establishing and maintaining the coor-
dinate framework (horizontal and satellite geodetic control networks), elevation
framework (vertical control network), and gravimetric framework (gravity control
network).
Classical horizontal and vertical datums are realized by classical geodetic
methods. Due to their limited controlling area, these two datums can only be used
as regional datums and are usually applicable countrywide. The three-dimensional
coordinate datums and gravity datums can be used as both global and regional
datums. The datums are represented by the position coordinates, heights, and
gravity values at a series of control points. To be specific, the datums are realized
through establishing different geodetic control networks, i.e., the extensions of
horizontal and vertical datums, three-dimensional coordinate datums, and gravity
datums are realized by the horizontal and vertical control networks, satellite
geodetic control networks, and gravity control networks, respectively.
The horizontal coordinates and heights of points on the Earth's surface deter-
mined by classical geodetic techniques refer to different datums. For instance,
methods such as triangulation and traversing can only obtain the two-dimensional
horizontal coordinates (x, y)or(L, B) of the surface point, whereas height H of the
point can only be obtained by leveling or trigonometric leveling. Because the