Geoscience Reference
In-Depth Information
Unified Specifications and Standards
China has a relatively large territory, so it costs a great deal of manpower, material,
and financial resources to build the national control network, which has to be
accomplished with the concerted efforts of many departments and agencies. Once
established, some parts of the geodetic network still need to be reinforced,
upgraded, and improved according to the needs of different sectors. To avoid
redundancy and waste of resources and to encourage sharing of benefits, the work
plan for network establishment and the specific operational standards should be
unified so that the accuracies of the results obtained by different surveying and
mapping institutions and the specifications of design meet the requirements and
such results will become an integral unity as a component of the national control
network. Issues like the overall plan, the anticipated target of precision, datum
selection, and so on should be in compliance with the technical standards. In
addition, such issues as specific implementation plans, instrument utilization,
operation methods, tolerance restrictions, and project checking and acceptance
should also be provided in the specifications.
3.1.4 Plans for Establishing a National Control Network
The China national horizontal control network built in the 1950s used primarily the
method of triangulation, but traversing was also employed in harsh areas. Below is
an overview of the establishment of the triangulation chains of various orders.
First-Order Triangulation Chain
The first-order triangulation chain is a national primary network, used to build a
precise framework of a unified coordinate system throughout the country to control
the establishment of the second- and lower-order triangulation networks and pro-
vide data for studying the size and shape of the Earth and geodynamics. Mapping
control is not the direct objective—accuracy has more importance in this case.
The first-order triangulation chain runs along the meridian and the parallel as
shown in Fig. 3.5 . The triangulation chain between the intersections is called the
chain section; the circle formed by the east-west and north-south chain sections is
called the chain loop; many chain loops form the chain system. The chain section is
approximately 200 km long and is usually formed by single triangles and may also
include some geodetic quadrilaterals or mid-point polygons. The average side
length of triangles in the chain ranges from 20 to 25 km, any arbitrary angle of
triangles is not less than 40 , and the distance angles of the geodetic quadrilaterals
or mid-point polygons should be greater than 30 . Computed by the triangle
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