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Introduction to Geodesy Ch1

Chapter one
Introduction to Geodesy
The classical definition of geodesy [pronunciation: jee-od-uh-see] is that it is the
science concerned with the shape, size, orientation and the gravity field of the Earth.
Merriam-Webster: a branch of applied mathematics concerned with the
determination of the size and shape of the earth and the exact positions of points on
its surface and with the description of variations of its gravity field
Geodesy aims at the determination of the geometrical and physical shapes of the
earth and its orientation in space. In general, geodesy is discipline that deals with the
measurement and the representation of the earth, including its gravity field in a 3D
time varying space.
Based on the concept of geodesy stated above the objective of geodesy with respect
to the planet Earth may be described as follows:
“The objective of geodesy is to determine the figure and external gravity field of the
Earth, as well as its orientation in space, as a function of time, from measurements
on and exterior to the Earth’s surface.”
Function of Geodesy
 To determine accurate position of a points
 To establish datum and coordinate system
 For mathematical projections
 To determine geophysical properties of the earth
 To state geodynamic nature of the earth
 Monitoring the solid Earth (displacement, subsidence or deformation of the
ground and structures due to tectonic, volcanic and other natural phenomena
as well as human activity);
 Monitoring variations in the liquid Earth (sea-level rise, ice sheets,
mesoscale surface topography features, mass transport);
 Monitoring variations in the Earth's rotation (polar motion, the length of
the day);
 Monitoring the atmosphere with satellite geodetic techniques (ionosphere
and troposphere composition and physical state);
 Monitoring the temporal variations in the Earth's gravity field;
 Determining satellite orbits (including Earth observation and navigation
 Determining positions - and their changes with time - of points on or above
the surface of the Earth with the utmost accuracy
Subdivision of Geodesy
Geodesy is usually subdivided into geometrical geodesy, physical geodesy,
and satellite geodesy:
Physical geodesy: is concerned with determining the Earth’s gravity field,
which is necessary for establishing height.
Geometrical geodesy: is concerned with describing locations in terms of
geometry consequently, coordinate systems are one of the primary products
of geometrical geodesy.
Satellite geodesy: is concerned with using orbiting satellites to obtain data for
geodetic purposes
Application of Geodesy
Applications of Geodesy in Topographic Mapping, Engineering Construction,
and Transportation
1. To control error accumulation in mapping. Errors are inevitable in
mapping, for instance, they arise when we depict a line of direction or
measure a certain distance. They are hardly noticeable in small areas, but
would gradually propagate and accumulate in mapping of large areas,
greatly deviating the topographic positions and features on a map. If a
geodetic network is used as the basis for mapping control, errors can be
constrained between adjacent control points to avoid accumulation and
propagation so as to ensure mapping accuracy
2. To unify coordinate systems. National basic topographic maps are
generally mapped subdivision by different departments at different stages
in different places. Because the coordinate system of points in the geodetic
control network is unified nationwide with homogeneous accuracy, missing
or overlapping layers do not occur in mapping, ensuring a perfectly fine
splice of adjacent map sheets to form an integrated map
3. To resolve conflict between an ellipsoid surface and a plane. A map is flat,
the Earth is approximately like a rotating ellipsoid with a non-developable
curved surface that would crumple or split if forced to flatten, indicating
one cannot directly map the features from an ellipsoid surface onto a plane.
However, the positions of geodetic control points on an ellipsoid can be
projected onto a plane via certain mathematical methods. Mapping on the
plane can therefore be controlled according to these point positions on the
4. To build a mapping control network for large-scale topographic mapping
at the project design stage. Designers design buildings and plan districts on
large-scale topographic maps. Geodesy serves to establish the mapping
control network as the basis for mapping control
5. To build a construction control network during project construction.
Construction survey is mainly used to set out the designed buildings on a
map and make sure that they are built in the intended locations. With
different projects, the concrete tasks of construction surveys differ. For
example, the major task of a tunnel construction survey is to ensure that the
tunnel dug from reciprocal directions runs through in accordance with the
specified accuracy. During layout, the direction and distance of the installed
instruments are both calculated on the basis of the control network, and
thus the construction control network has to be established beforehand with
the required accuracy
6. To build a special control network for observing deformation with the
purpose of monitoring deformation of buildings during the operation stage
of the project
7. Boundary demarcation
8. Hydrology
9. Urban Management
10.Engineering projects: Road, Dam and any building construction ...etc.