Geodetic Datum and Geodetic Control Networks - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 92

accidently disturbed, the leveling lines should be well away from the cities, railway
stations, or other built-up areas, and be free from terrain obstacles like rivers, lakes,
or valleys. It should be noted that when the intended leveling routes coincide with
the old leveling lines, the old benchmarks should be made the best use of, if
possible. New benchmarks should otherwise be set and connected to the old
benchmarks.
Site Selection
After drawing up designs, reconnaissance surveys should be carried out to select
routes and identify the site of benchmarks. The purpose of reconnaissance and route
selection is to enable the design to accord with actual situations and to determine
the viable leveling routes and benchmark sites. Considerations for selecting the
benchmark sites include stability, security, long-term preservation, and accessibil-
ity. The benchmark sites should not be selected in areas vulnerable to floods, in soft
soils, low-lying regions, or in places subject to fault movements. Locations where a
benchmark cannot be found or be conveniently leveled to should also be avoided.
Monumentation
Benchmarks and identifiers will be used to mark the benchmark sites determined by
reconnaissance survey over a long time to meet the needs of leveling connection.
Benchmarks can be sorted into three categories according to their purposes: fun-
damental benchmarks, ordinary benchmarks, and benchmarks in bedrock. Their
uses and underlying characteristics will be discussed separately below.
The fundamental benchmark serves to secure precise leveling results over a long
time, in support of connection to the newly established benchmarks or for checks
and renewals of elevations of previously established benchmarks. Primary bench-
marks are established on the first- and second-order leveling lines, placed at
intervals of 20-30 km and at approximately 60-km intervals in harsh desert areas.
The ordinary benchmark serves to provide an elevation reference for topo-
graphic and other engineering surveys directly. They are established on leveling
lines of various orders at intervals of 4-8 km in general.
The benchmark in bedrock is a permanent benchmark directly anchored to the
outcrop and is the primary basis for studying vertical crustal and surface motions. In
general cases, the benchmark spacing on the first-order leveling lines is specified at
500 km. The number of benchmarks in megacities or near seismic zones can be
increased to meet the requirements of scientific research.
Figures 3.17 and 3.18 show the specific forms, specifications, and installations of
the primary and ordinary benchmarks.
After the leveling route design, benchmark site selection, and monumentation,
we can obtain the height of the benchmark on the Earth's surface by the method of
leveling.