Geodetic Datum and Geodetic Control Networks - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 75

The national horizontal control network in China is classified into four orders.
The first-order triangulation chain with high accuracy and low density crisscrosses
the whole country to form the key network of the unified coordinate system. Then,
the second-, third-, and fourth-order horizontal control networks are established in a
piecemeal manner according to actual needs. Special requirements are with respect
to the following factors.

Sufficient Accuracy

Apart from being the control framework of the national unified coordinates, the
first- and second-order networks, in the process of establishing the national hori-
zontal control network, have to meet the requirements for mapping of the basic
scale topographic maps and the development of modern technology, such as space
technology, precise engineering, earthquake monitoring, and geodynamics,
whereas the third- and fourth-order horizontal control networks are used chiefly
for a higher-level control of the topographic mapping control points and to satisfy
the needs of fundamental engineering construction. Control points of various
orders, therefore, must cater for the actual demands. For example, the accuracy of
the first- and second-order control points should meet the needs of a 1:50,000 scale
topographic map, while that of the third- and fourth-order control points should
meet the needs of topographic mapping at a scale of 1:10,000.

Necessary Density

Density of the control points in the control network means that there is usually one
single point every several square kilometers on average. It can also be expressed by
the average side length of midpoints in the control network. The shorter the side
length, the denser the geodetic points will be. The controlling area Q of each point is
expressed by the average side length S, namely:

S 2 , Q ¼

ð 3 : 1 Þ

and

p ,

S ¼

which is the relationship between the side length and the controlling area.
The density of the points is required to be different according to different
mapping scales and methods. On average, three or four geodetic points are gener-
ally required to densify control points for each map sheet. For different engineering
projects, however, the desired density of points will presumably be different and
should be determined according to real situations.