Introduction - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 4

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 prop-
agate and accumulate in mapping of large areas, greatly deviating the topo-
graphic 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 subdivisionally 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, but
the Earth is approximately like a rotating ellipsoid with a non-developable
curved surface that would crumple or split if forced to flatten, indicating that
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 plane.
In this case, geodetic control points with certain densification have to be
established first for topographic mapping. Traditional geodetic surveying has
lower efficiency, consumes more time, and requires a greater workforce and a
huge investment. With the fast-paced development of the economy, the demands
for various kinds of medium- and large-scale maps are increasing rapidly, meaning
that quick and precise positioning and rapid mapping techniques are needed to
provide a guaranteed product. Modern Global Navigation Satellite Systems
(GNSS), such as a Global Positioning System (GPS), can locate the position of a
point within 5-10 min (compared with several hours to days using traditional
methods) with centimeter-level accuracy. GPS allows rapid large-scale mapping
when used for aerial photography and surface auto-mapping systems.
In engineering construction, the important roles of geodesy are:
1. 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.
2. To build a construction control network during project construction. A construc-
tion 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