ISPRS, Vol.34, Part 2W2, '‘Dynamic and Multi-Dimensional GIS", Bangkok, May 23-25, 2001 
system management and construction. Moreover, the 
construction of duct system belongs to underground 
engineering, which has a feature of invisibility. To repair and 
survey the pipelines and position the valves by means of 
routine surveying technologies will cause more and more 
errors. As a result, the problems cannot be solved and will 
leave a legacy of trouble in the future. Therefore, according to 
the requirements of urban development as well as the 
management and construction of duct system, it is critical to 
use the 3S technology to obtain the actual materials for the 
duct system rapidly, to strengthen the scientific management of 
the duct system and to construct people's lifeline with quality. 
2. RAPIDLY OBTAIN ACTUAL MATERIALS FOR DUCT 
SYSTEM BY APPLYING GPS AND RS 
As the duct system belongs to underground engineering with a 
feature of invisibility, pipelines and their topographic 
information are important references for the management and 
construction of duct system. No matter the planning, design 
and construction of the pipelines, or inspection, maintenance 
and other management of pipeline valves, they depend greatly 
on pipeline network and its topographic information. However, 
at present, most Urban Water Supply Department in China are 
in serious lack of accurate pipeline fundamental stuff - basic 
pipeline network map. Thus, it is urgent for the Urban Water 
Supply Departments to obtain basic pipeline network map with 
actualization. 
Commonly, it is to obtain the fundamental materials of pipeline 
network by adopting routine surveying methods. However, it is 
hard to realize due to many reasons. At first, the surveying 
control points of the pipeline network map are short. It is of low 
precision by adopting routine technologies on one hand. And 
on the other, as the accumulated error is large, it will have an 
execrable effect on the construction and management in the 
later stage. Secondly, to establish a set of surveying control 
network for the urban duct system with routine technologies is 
a long-time and formidable project, which is neither easy to 
realize nor economical. Thirdly, as the urban infrastructure is 
developing rapidly, the construction and reconstruction of the 
duct system is stepping faster, it is difficult to catch up with the 
speed of pipeline construction by means of routine surveying 
technologies. As a result, it is unable to guarantee the 
actualization of fundamental materials for the duct system. 
Therefore, it is exigent to adopt advanced surveying 
instruments and facilities to enhance the surveying and 
repairing abilities for the pipelines overall. 
Since the American government has decreased GPS 
positioning precision factitiously to restrict the GPS users 
without privileges, the Chinese GPS users cannot receive P 
code and the real-time positioning precision has decreased to 
about 100m. To use GPS with common application mode is 
hard to give full play to its advantages. In order to counteract 
the SA policy of the American government and reduce the 
resource waste and low efficiency caused by using routine 
differential GPS repeatedly, it is necessary to develop 
differential GPS technology and improve GPS real-time 
positioning precision. After great efforts of three years, Wuhan 
Technical University of Surveying and Mapping (WTUSM), 
Chinese Scientific Research Institute of Surveying and 
Mapping (CSRISM) and National Fundamental Geographic 
Information Center (NFGIC) has jointly worked out a holistic 
scheme to establish Chinese Wide Area Differential GPS 
Network (WADGPSN) and improve GPS real-time positioning 
precision. The real-time positioning precision can be increased 
to the level of decimeter and even centimeter. Therefore, the 
repairing and surveying of underground duct system can be 
realized by means of Local Area Differential GPS (LADGPS) or 
Wide Area Differential GPS (WADGPS) real-time positioning 
precision technologies. 
LADGPS is achieved on the base that the base station and 
user station in a certain distance track GPS satellite 
synchronously in the same orbit. Generally speaking, if the 
base station and user station observe the same satellite 
synchronously and they are within a certain distance (generally, 
not beyond 150km), the observation values of the two stations 
belong to the same orbit arc of the same satellite. A strong 
relativity exists between the two stations, i.e., they have the 
same error. Thus, take the false-distance differential as an 
example. Transfer the modified information (false-distance 
modified variability information should be added generally) 
according to false-distance scale of the false-distance error of 
each GPS satellite observed by the base station to the 
neighboring user station through data communication chains. 
User station will modify its false-distance with the help of the 
information and obtain the real-time positioning precision of 
+(3-10)m.