Establishment of Geodetic Coordinate Systems - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 365

the origin point to another point and the normal height H normal0 at the geodetic
origin, we can obtain L 0(1980) , B 0(1980) , A 0(1980) , and H 0(1980) as the geodetic origin
data of the XAS80.
XAS80 has adopted JYD 1968.0 instead of CIO as the origin of the polar coordi-
nate (where JYD is the acronym of Chinese Pinyin Ji Yuan Dian, meaning polar
origin). This polar coordinate system was established in 1977 by the Polar Motion
Collaboration Group in China through analysis and study of the long-term and
periodic components of the Earth's pole using the latitude survey data from the
optical instruments of 36 sites abroad over the years 1949-1977. Participants of the
Collaboration Group include the Department of Astronomy, Nanjing University,
Shanghai Astronomical Observatory, Shaanxi Astronomical Observatory,
Wuchang Time Observatory, and Tianjin Latitude Observation. The system has
been maintained by Tianjin Latitude Observation using follow-up latitude survey
data from basically the same sites and instruments, and the same methods for
mathematical data processing.
We have to acknowledge that the precision of JYD 1968.0 is fairly high in terms of
determining a polar system by means of optical techniques. The system is also quite
stable. Its internal accuracy is equivalent to the CIO of BIH, but the average
deviation of the external consistency is less than 0. 02 00 compared with BIH
and IPMS.
Although JYD 1968.0 is used in XAS80 in China, there have always been disputes
over its employment in Chinese activities in the fields of astronomy and geodesy. In
particular, with the improvements in space geodetic techniques, the traditional
optical astrometric methods have been replaced by new techniques. The JYD
system is a polar system maintained by optical means, and thus it cannot fit in
with the needs of establishing a high-precision reference system in modern society.
In addition, JYD is not a truly independent polar system and the optical latitude
survey data in overseas countries are on the brink of exhaustion. Consequently, this
system can barely be maintained. Changing the polar origin to make it consistent
with the international system is therefore inevitable.
We also have to make clear that the ellipsoid is oriented to minimize the sum of
the squared height anomalies within China by the XAS80 based on the ellipsoidal
parameters recommended by IUGG1975, which means that only the three
unknowns Z 0 in ( 7.35 ) have to be solved. If the ellipsoidal

parameters are not fixed in advance, finding solutions based on ( 7.35 ) will result
in

Δ X 0 , Δ Y 0 , and Δ

e 2 , i.e., five parameters. It is evident that such an
orientation can allow the ellipsoid surface to be a closer fit to the geoid (or quasi-
geoid) in China. As indicated by several computational results over the period
1964-1977, the value of a ranges from 6,378,666 to 6,378,685 m and that of f from
1:291.6 to 1:292.2. It can be seen from the above that the semimajor axis of the
calculated ellipsoid based on the regional geoid in China is 500 m longer than that
of the obtained ellipsoid based on a global geoid at present. Their denominators of

Δ X 0 , Δ Y 0 , Δ Z 0 , Δ a, and Δ