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these stations, the China Gravity Basic Network 1957 (abbreviated as CGBN57)
took its shape. The error of its base station relative to the Beijing gravity datum was
no more than
10
5
m/s
2
, and the error of the first-order stations was no
0.32
10
5
m/s
2
. The datum of this network has been introduced
from three base stations of the former Soviet Union and belongs to the Potsdam
Gravity System.
Nearly three decades since the establishment of the CGBN57, the departments
concerned have measured tens of thousands of gravity stations of different orders.
These gravity points are crucially important in national economic construction and
national defense building.
In the early 1970s, the Chinese Academy of Metrology successfully developed
the free-fall absolute gravimeter and initiated the first absolute gravity determina-
tion in China. Connected to the Beijing gravity origin, the observation proved that
the original value was 13.5
more than
0.40
10
5
m/s
2
too high. Therefore, in operations, once
the Potsdam Gravity System is adopted,
10
5
m/
the correction of
13.5
s
2
should be applied with no exceptions. Some departments add
10
5
m/
s
2
to the gravity value of the Potsdam Gravity System according to the resolution of
relevant international organizations.
14.0
China Gravity Basic Network 1985
The major problem of CGBN57 lies in that it has no absolute gravity station
(generally known as a reference point or datum). Passing through many places,
the gravity system was tied to Potsdam. At that time, the accuracy of the relative
gravimeter was rather low. The Potsdam Gravity System was superseded by
IGSN71 but China did not adopt this new system. Therefore, establishing a
second-generation national basic gravimetric network was indispensable.
In 1981, China and Italy jointly measured 11 absolute gravity stations in China
involving the use of free-fall absolute gravimeters from the Italian Institute of
Metrology in accordance with the Sino-Italian Agreement on Scientific and Tech-
nological Cooperation. Under the organization of the State Bureau of Surveying
and Mapping (SBSM), and with the participation of the Department of Geology and
Mineral Resources, Department of Petroleum Resources, China Seismological
Bureau (CSB), State Bureau of Metrology, Surveying and Mapping Bureau of the
General Staff Headquarters of the Chinese People's Liberation Army (SMBGSH),
Institute of Geodesy and Geophysics affiliated to Chinese Academy of Sciences
(CAS), and some other departments, a connection survey of the national gravimet-
ric basic network was conducted between 1983 and 1984. The overall surveying fell
into two stages, employed nine gravimeters of LCR-G model, and was conducted in
accordance with the “National Gravimetric Basic Network Field Operation Regu-
lations (for Trial Implementation).” In terms of a relative connection survey, there
should be at least four results from two instruments for each survey line and the
mean square error of the average value should be no greater than
10
5
m/s
2
.
Meanwhile, six LCR-G model gravimeters were employed to conduct
15
the
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