Geoscience Reference
In-Depth Information
Table 3.6 Overview of China gravity networks
Measurement precision
(10
8
ms
2
)
Number of points
Datum
point
Basic
point
I-order
point
Datum
point
Basic
point
Name
System
CGBN57
21
82
150
Potsdam system
CGBN85
6
46
10
20
IGSN71
CGBN2000 21
126
5
10
Absolute gravity
system
3. Establishment of the gravity stations should give consideration not only to the
needs of economic growth but also to the needs of national defense building and
protection against and mitigation of earthquake disasters.
4. The network structure of the connection lines should be optimally designed.
5. The newly established gravity network stations should be connected to the old
stations as well as the basic network of the crustal movement observation
network of China (Fig.
3.26
).
China has successively established the CGBN57, CGBN85, and CGBN2000
based on the above principles. Our discussions below will focus only on the plans
for establishing regional gravity networks.
Plans for Establishing China National Gravity Networks
Gravimetry can be classified into two categories based on the purpose and accuracy
of gravity measurements, i.e., “gravity control measurement” and “densifying
gravity measurement.” The former serves to establish control networks that include
basic gravity points, first-order gravity points, and second-order gravity points. The
latter, on the other hand, is conducted based on the gravity control point fulfilling
the needs of the special tasks of various units and departments. Methods of
connection survey and instruments used in gravity measurement vary with gravi-
metric order. This will be discussed further.
National gravity control networks should be established first before conducting
gravity measurements in a country or region to obtain detailed gravity field data. A
control network provides starting data for gravity measurements in areas of interest
and helps with control of gravimetric error accumulation. As stated previously,
China has successively established the CGBN57, CGBN85, and CGBN2000, which
have offered the initial datum to perform nationwide relative gravity measurements.
China has a vast territory, so the very few gravity control points of the CGBN2000
are apparently insufficient. It is hence necessary to extend further the first-order
gravity network on the basis of the CGBN2000.
A first-order gravity point begins at a basic gravity point of known control in
CGBN2000, the multi-line being connected pointwise to several unknown points
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