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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004 
When the tracing tour comes back to either end of this segment, 
the tracing stops. Because the possible polygons to include the 
segment may exist in both sides of it, the tracing process is 
executed in both clockwise and counter-clockwise directions. 
After polygon breaking, many short branches remain besides 
the axes. Therefore, at this stage called Branch Reduction, all 
the branches are reduced a length of half of the profile length 
(Figure 10). The actions taken for this process are: (1) all the 
end-points are located, (2) the connections of those end-points 
are eliminated, and (3) round = round + 1; if the round is less 
than the (profile length) / 2; step (1) is repeated. 
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Figure 10. Branch reduction 
3.4 Line Smoothing 
This process simply takes the position weighting average of the 
target and its connected neighbors as its new position (Figure 
11). The weight of each target is a value proportional to 
elevation for ridges, inversely proportional to elevation for 
valleys. The weight of each target is a value proportional to its 
elevation in the ridge case or inversely proportional to its 
elevation in the valley case. Because the new position is an 
average of the neighboring points and itself, the shifting 
distance is never more than a grid interval. 
  
Figure 11. Line smoothing 
3.5 The Valley Axes Case 
When the target lines are the valley axes, the PPA program 
simply reverses the topography prior to the recognition 
procedures. 
The program is straightforward to use because only the target 
type and the length of profile recognition need to be defined by 
the user. Once the data and these two parameters are given, the 
program can be left to run on its own. 
621 
4. EXPERIMENTAL TESTING 
Experimental testing is realized for comparing the methods 
developed for deriving the skeleton lines from contours and 
DEMs. Firstly, skeleton lines are derived from contours (Figure 
12), which are generated from the height points of the DEM 
(Figure 13) in addition to the characteristic points of the terrain, 
in accordance with the method developed by Aumann et al. 
(1991). The DEM covers an area of 650 m x 500 m in a map of 
Turkey at scale 1:5000 (Figure 12). The height points are 10 
meters apart and the DEM consists of 51 rows and 66 columns. 
After that, the algorithm developed by Chang et al. (1998) for 
derivation of the skeleton lines from the height points of the 
DEM is applied. In this study, as the length of profile, it is not 
assigned any value. In other words, all points are assumed as 
the target points. It should also be noted that two methods uses 
the same data in fact, because the contours are also generated 
from the height points of the DEM. 
  
     
  
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Figure 12. Contours generated from the height points of the 
DEM in addition to the characteristic points of the 
terrain 
   
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Figure 13. Height points of the DEM 
Figure 14 shows the skeleton lines drawn by the operator in the 
stereo model. The results of the derivation process, i.e. derived 
skeleton lines in accordance with the methods developed by 
Aumann et al. (1991) and Chang et al. (1998) (right hand side), 
are represented in Figure 15 and 16 respectively.