Yandong Wang 
  
3.3 Removal of Non-Road Lines 
After grouping of lines, line segments are connected to form continuous roads. At the same time, some non-road line 
features still exist together with roads. These non-road features need to be removed based on some properties of road 
networks before a complete road network is formed. It is well known that a road rarely exists individually, but connects 
to other roads to form a network. Some topological properties of road networks in an image can be represented by the 
following rules: 
Rule one: if a line passes through two different edges of the image, then it is a road. 
Rule two: if one end of a line connects to a road and the other passes through an edge of the image, then it is a road. 
Rule three: if one end of a line connects to a road and its length exceeds a given threshold, then it is a road. 
Other rules may also be developed according to the local characteristics of road networks. Using the above three rules, 
isolated and short line segments can be removed from the linked line images and true roads are retained. 
  
Figure 2. Hierarchical grouping of line images a. an aerial image b, c, d. line images generated using thresholds of 5, 
10 and 20 pixels. 
  
946 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000.