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Figure 5. Reduced branches at scale 1: 5000 
  
  
  
  
  
Figure 6. Smoothed branches at scale 1: 5000 
In the first step, after the processing of ‘Counter’ subroutine 
calculated branches lengths are sent to ‘BranchReduction’ 
subroutine. The branches of one segment are deleted. In the 
second step, ‘Counter’ subroutine are repeated and it calculates 
new lengths of branches, then ‘BranchReduction’ subroutine 
deletes branches if they are one or two segment in length. This 
process is repeated until any branches that are shorter than the 
threshold value do not remain. 
After the process, segment coordinates can be saved in a text 
file as the points of terrain skeleton lines. On the other hand, all 
the following segments groups are not smooth enough for map 
representation. They indicate sharp turnings that rarely occurred 
in land forms. To solve the problem, line smoothing step is 
added to *RidgeValleyAxisPicker' program. 
3.4 Line Smoothing 
The process can result in targets moving to positions that better 
match the line trend. Because the new position is an average of 
the neighboring points and itself, the shifting distance is never 
more than a grid interval. In this way, the process does not 
violate the information provided by the gridded data set (Chang, 
et al., 1998). Figure 6 shows the results of line smoothing of 
lines in Figure 5. 
4. CASESTUDY 
In case study, *RidgeValleyAxisPicker' has been used to extract 
terrain skeleton lines. All of the DEM points are automatically 
collected from a stereomodel in 10 meter grid distance for the 
map at the scale 1: 5000 (Figure 7). The points are saved in a 
text file with their row and column numbers (ie. grid 
coordinates) and their elevations. Grid coordinates are obtained 
from land coordinates by means of a transformation. 
Figure 7. 10m gridded DEM points sample at the scale 1: 5 000 
Figure 8 shows the extracted and the smoothed ridge and valley 
line results of the *RidgeValleyAxisPicker' program for the 
whole study area. 
Although the method for automatic extraction of terrain 
skeleton lines can be working with a huge amount of data quite 
rapidly, there are some cartographic problems on the extracted 
lines. The results of ‘RidgeValleyAxisPicker’ program are 
evaluated with the harmony of extracted skeleton lines and 
contours. Figure 9 shows the result of digitized valley lines for 
the same topographical map. The valley lines are only 
demonstrated in these examples, because ridge lines are not 
usually digitized for topographical maps. 
The numbers of the digitized valley lines and the program 
results are different because of the effect of branch reduction 
step. Some short valley lines are reduced and not demonstrated 
in Figure 8 as a result of branch reduction step. However, there 
are some unexpected valley lines on hill sides and no valley line 
in valley region. 
Moreover, the extracted and smoothed valley lines do not bend 
precisely on contours. Figure 10 shows this condition clearly. 
Many of the extracted valley lines are not entirely appropriate 
to contours.