ie 
  
1. Target Recognation 
+  “ConnectAI’ 
+  ‘SortSegment’ 
  
  
  
Ÿ 
2. Polygon Breaking 
+  '"PolyzonBreaking 
' 
3. Branch Reduction 
+  “BranchReduction’ 
* ‘Counter’ 
  
  
  
  
  
  
4. Smoothing 
NO Nag ng de YES 
* “‘NSmoothOut * "SmoothiOnut' 
« “Smooth” 
  
  
  
  
  
  
  
  
  
  
v Ÿ 
3. Saving the segment coordinates 
  
  
  
Figure 2. The flowchart of ‘RidgeValleyAxisPicker’ program 
3.2 Polygon Breaking 
Terrain skeleton lines are like a network structure and never 
closed polygon according to Chang et al. (1998). On the other 
hand, after the ‘ConnectAll’ subroutine, line segments are 
closed structure in Figure 3. In order to solve closed polygon 
and obtain a network structure, it is applied the 
‘PolygonBreaking’ subroutine to the sorted segments after the 
target recognition step automatically. The polygon breaking is 
a repeated procedure to check the closed polygon and eliminate 
the least important segment within each polygon until no closed 
polygon of any size remains (Figure 4). The importance of the 
segments is determined by their weight related to elevation 
value relatively. For example, for ridge depiction, the segment 
with lower elevation is less important than segments with 
higher elevation. 
3.3 Branch Reduction 
In order to extract more accurate skeleton lines, the effect of the 
branch reduction step is modified in this study compared to 
previous study of Chang et al. (1998). For this purpose, short 
segments that do not reflect to terrain characteristic are deleted 
iteratively in order to obtain proper branches that are same as 
the terrain skeleton lines for topographical map (Figure 5). 
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004 
  
Figure 3". Connected points and sorted segments at scale 
1: 5000 
  
  
  
  
Figure 4. Solved closed polygons and new segments at scale 
1: 5000 
Short branches to be deleted are determined in accordance with 
the threshold value depending on the grid distance and map 
scale. In order to obtain the proper branches, 
‘BranchReduction’ and ‘Counter’ subroutines are used in 
‘RidgeValleyAxisPicker’ program. ‘Counter’ subroutine 
calculates branch lengths from end points to the node points. 
  
* 20m. grid distance used because of the easy showing segments 
characteristics 
   
  
   
     
  
   
    
     
   
   
  
   
   
   
    
  
  
       
   
  
  
  
  
  
  
   
  
   
     
   
   
       
        
   
   
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