International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004 
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Figure 5. Automatically generated Tie/Pass Points (a), (c) extracted tie/pass points before removal of shadow points, (b), (d) 
extracted tie/pass points after removal of shadow points 
A visual check of the generated tie/pass points by ISAT reveals 
that more than 80% shadow/tree points were removed in this 
project. About 20% non-shadow points were treated as shadow 
points and removed incorrectly. But the algorithm created some 
other tie/pass points automatically to replace them and there is 
no effect to the final results of bundle adjustment. Figure 6 
shows the distribution of tie/pass points generated by ISAT 
before and after removal of shadow points. It can be seen that 
plenty of well-distributed tie/pass points were generated after 
removal of shadow points although the total number of tie/pass 
points is reduced slightly. 
4, CONCLUSIONS 
In automatic triangulation, generation of good tie/pass points is 
crucial to the final results of bundle adjustment. The existence 
of shadow points may reduce the accuracy and reliability of 
triangulation, especially when the block is very large, the sun 
azimuth is large or photos are taken in bad weather conditions. 
In this paper, a new method based on the properties of shadow 
points is presented. The method detects shadow points in two 
steps. First, it produces the candidates of shadow points by 
checking the image gradient and then determines shadow points 
by checking slope change in a local area around the selected 
candidates. The test results have shown that shadow points can 
be removed effectively by the proposed approach during 
automatic generation of tie/pass points. 
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Figure 6. Distribution 
within strips