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international connection survey in Beijing, Shanghai, Paris, Tokyo, Kyoto, and
Hong Kong. This has made possible the interconnection between the China national
basic gravity networks and the international absolute gravity base station, IGSN71
station, as well as the international gravity tie points of the Japan Pacific Rim.
Adjustment was made in 1985 by the Chinese Academy of Surveying and
Mapping, affiliated to the SBSM. The known base stations were Beijing, Shanghai,
Qingdao, Fuzhou, Nanning, and Kunming. Considering the uneven distribution of
these six base stations, the largest range of gravity only accounting for 65 % of the
gravity range of the whole network, another five gravity points were used as the
known points—Paris, Tokyo A, Tokyo B, Kyoto, and Hong Kong. The number of
known gravity points was 11 in total. During adjustment, the observational values
involved the conversion of recorded data from instruments and the corrections of
instrument height, solid Earth tide, and atmospheric pressure. It was conducted by
means of indirect adjustment of unequal weight. The China Gravity Basic Network
1985 (CGBN85) consists of 6 datum points, 46 basic points, and 5 points derived
from the basic points. It has not only improved the graphical structure and offered
external precision criteria, but also made the CGBN85 and IGSN71 closely
interconnected, and brought the gravity system of CGBN85 into the system of
IGSN71.
After overall adjustment, the mean square error of weight unit of the CGBN85
was
10
8
m/s
2
. The mean square error of the gravity value of the point
(internal consistency) was
15
10
8
m/s
2
. After checking the
external consistency, the gravity values were found to be subject to some systematic
effects. Therefore, the accuracy of the gravity values of the CGBN85 has been
estimated to be somewhere between 20
10
8
m/s
2
to
8
13
10
8
m/s
2
.
CGBN85 is the second national gravity control network in China, including a
basic network and a first-order network. Its gravity datum has been jointly defined
by the observed values obtained using many domestic absolute gravimeters and the
known international gravity system. It must be pointed out that some of the datum
values of the international gravity points are the observed values of absolute
gravity, some are from the IGSN71 system, and some are the tie-in observations
of the Japan Pacific Rim. Now we can see with hindsight that the gravity datum of
this jointly defined CGBN85 was not defined independently by the absolute gravity
points in China. It can only be described as something combined or synthetic.
10
8
m/s
2
and
30
China Gravity Basic Network 2000
The accuracy of the CGBN85, as opposed to the CGBN57, was improved by one
order of magnitude, eliminating the error of the Potsdam System and increasing the
density of basic points. China's basic gravity control network has played a crucially
important role in various fields like surveying and mapping, geology, seismology,
petroleum, and national defense since its implementation over a decade ago.
However, with the passage of time, the economy has seen rapid growth, often
making the basic points of the CGBN85 awkward or impossible to use. A survey
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