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WGS84 also includes the reference ellipsoid, fundamental constants, an Earth
gravity field model, and a global geoid model (see NIMA 2000; NGA 2002).
The WGS84 reference frame was realized by a globally distributed set of
monitoring stations. To establish the WGS84 reference system that agreed with
ITRF, the USA refined WGS84 three times in 1994, 1996, and 2001, respectively. A
refinement to the WGS84 in 2001 included data from 49 IGS stations and the station
coordinate set has been given the designation WGS84 (G1150) at the epoch 2001.0.
It was aligned to ITRF2000 with an accuracy of 1 cm in any coordinate component.
Hence, for most applications that require an accuracy of less than centimeter level,
WGS84 can be considered the same reference frame as ITRF.
For the four fundamental constants of the WGS84 Ellipsoid see Table 4.1 .
7.5 Geodetic Coordinate Systems in China
7.5.1 Beijing Coordinate System 1954
The Beijing Geodetic Coordinate System 1954 (abbreviated as BJS54) is virtually
the extension of the former Soviet Union Pulkovo Coordinate System 1942 in
China. Thus, the latter will be discussed first.
Prior to 1946, the former Soviet Union adopted the Pulkovo Coordinate System
1932. Its geodetic origin was located in the center of the Great Round Hall at the
Pulkovo Astronomical Observatory. It used the Bessel ellipsoidal parameters,
oriented by a single astronomical position, only the initial geodetic azimuth was
obtained through adjustment by using 45 Laplace's azimuths in the astro-geodetic
network.
Such a coordinate system was not suitable for the former Soviet Union. First, the
semimajor axis of the Bessel ellipsoid was too small, with an error of approximately
800 m. Second, the positioning and orientation of the reference ellipsoid was not
well suited. In the Far East regions, the deviation of the geoid from the reference
ellipsoid was up to 410 m. Therefore, the Pulkovo Coordinate System 1942 was
established based on the Pulkovo Coordinate System 1932, but using parameters of
the Krassowski Ellipsoid. The geodetic origin was still at Pulkovo, but the astro-
geodetic orientation was performed this time.
During the 1950s, in the early phase of the establishment of an astro-geodetic
network in China, to accelerate the cause of surveying and mapping and fully
develop mapping, we were in desperate need of a geodetic coordinate system.
Thus in 1954, with the advice and support provided by the parties concerned, and
considering the historical condition at that time, the Surveying and Mapping Bureau
of the General Staff Headquarters of the Chinese People's Liberation Army
(SMBGSH) connected the first-order triangulation chains in China with the first-
order chains in the Far East of the former Soviet Union. Using the former Soviet
Union 1942 Pulkovo coordinates at the shared part of two baseline networks in
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