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flattening differ from each other by approximately 6 units due to the complex
terrain environment in China. As a result, the ellipsoid parameters based on China's
regional geoid have not been adopted in the XAS80.
Highlights of the XAS80 can be summarized as:
1. It is a local coordinate system.
2. It has adopted four ellipsoidal parameters, including both geometric parameters
and physical parameters. The numerical values are those recommended at the
16th general assembly of IUGG held in 1975 (see Table
4.1
).
3. It employs astro-geodetic positioning and orientation. We select 922 points
evenly throughout the country at a 1
o
1
o
interval and form an arc measurement
equation;
the values of
ʾ
0
,
ʷ
0
, and
ʶ
0
solved at
the geodetic origin are
1. 9
00
,
1. 6
00
, and
14.0 m.
4. It has a well-defined orientation. The minor axis of the Earth ellipsoid is defined
as parallel to the direction from the Earth's center of mass to the polar origin
JYD
1968.0
, and the planes of the initial geodetic and astronomical meridians are
parallel to each other in China;
ʾ
0
ﾼ
ʷ
0
ﾼ
ʶ
0
ﾼ
0.
5. The origin is located at Yongle township, a small town 60 km north of Xi'an,
Jingyang county, Shaanxi province, in the central area of China, where the
precisions of estimated coordinates are relatively homogenous. It can be abbre-
viated as the Xi'an origin. The estimated values of geodetic longitude and
latitude are L
0
ﾼ
ʵ
X
ﾼ ʵ
Y
ﾼ ʵ
Z
ﾼ
34
o
32
0
.
6. The coordinates of nearly 50,000 stations are obtained by an integrated adjust-
ment of the nationwide astro-geodetic networks after the establishment of the
XAS80.
108
o
55
0
, B
0
ﾼ
The XAS80 demonstrates a marked superiority over BJS54. For example, it is in
complete conformity with the principles for establishing a classical local coordinate
system and can be easily accounted for; the number of the Earth ellipsoid param-
eters and the magnitude of values are more reliable and accurate; the pointing
direction of the coordinate axes is clearly specified; the ellipsoid surface fits the
geoid fairly well; and the nationwide mean value of geoid undulations drops from
29 m of BJS54 to 10 m, the maximum value appearing in the southwest of Tibet.
For most of the areas in the whole nation the geoid undulation is within 15 m
(cf. Fig.
7.9
).
In addition, the observational data are reduced rigorously and nationwide unified
integrated adjustment is carried out. Thus, the effects caused by the local adjust-
ment and unsuitable network control have been removed, and the accuracy of the
adjusted coordinates has been improved.
The changes in ellipsoidal parameters and orientation will inevitably cause
variations in geodetic coordinates. Admittedly, the positions of sheet lines will
also undergo changes, the degree of which will vary with the point positions. In
areas east of 102
east longitude in China, the maximum change is approximately
80 m, with an average of about 60 m. If the positions of sheet lines change, there
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