Geodetic Datum and Geodetic Control Networks - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 74

Fig. 3.4 Triangulation
measuring the angles, it will be called the method of combined triangulation and
trilateration or triangulateration.
3.1.3 Principles of Establishing a National Horizontal
Control Network
A national horizontal control network is a fundamental construction project. We
therefore need to proceed from the real situations of a nation, properly handle the
relationship between quality, quantity, time, and expenditure according to theory
and the real experience of network establishment, and work out specific principles
as the basis for designing and establishing the geodetic control networks.
Network Establishment and Control Based on Hierarchical Orders
A national horizontal control network can be established based either on a single
order or several different orders. The single-order control network can serve
directly as the basis for mapping control and is usually established in countries
with smaller territories to ensure more homogeneous accuracy throughout the
network and facilitate adjustment computations. Countries with vast territories
often adopt the method of establishing networks from higher to lower orders.
They usually first build a nationwide primary control network with higher accuracy
and lower density as a consistent control framework, and then continue to densify
the control network in a piecemeal fashion according to the needs of different areas.
The side lengths of control networks become shorter and the accuracies get lower as
the order changes from higher to lower. Using such a method to establish other
triangulation networks successively in different areas within one consistent coor-
dinate system can not only satisfy the desired accuracy but also achieve the
effective results at a faster pace and lower cost.