Geodetic Datum and Geodetic Control Networks - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 78

Fig. 3.6 Second-order
continuous network

Third- and Fourth-Order Triangulation Networks (Points)

National third- and fourth-order triangulation networks (points) can be further
densified on the basis of the second-order network, as illustrated in Figs. 3.7 and
3.8 . They are foundational to the mapping control survey and their density should
accord with the mapping scale. The average side length of the third-order triangu-
lation network is 8 km and the controlling area of each point is roughly 50 km 2 ,
which can basically meet the needs of 1:25,000 scale mapping. The average side
length of the fourth-order network is 4 km and the controlling area of each point is
around 20 km 2 , which can meet the needs of 1:10,000 and 1:50,000 scale mapping.
At each point of the third- and fourth-order networks there will be stations set for
observation. The mean square error of angle observation computed through triangle
closure should be less than

1.8 00 2.5 00

and for the third- and fourth-order

networks, respectively.

Traverse Control Network

Although traversing is not as effective as triangulation in controlling area, checking
conditions, and constraining error propagation of azimuths, it still has distinct
advantages that allow the network to be established in a flexible manner. Mean-
while, the survey can easily be carried out and terrain obstacles are more likely to be
overcome. In the early 1960s, the first- and second-order control networks were
sparsely established by traversing in most areas of the Tibetan Plateau. Traversing
has become increasingly widely used with the prevalence of total stations and the