The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Voi. XXXVII. Part Bl. Beijing 2008 
Figure 7. Edge buffer area formed in the aerial image 
The non-maximal suppression is the critical step of the Canny 
edge detector. If the step is carried out solely using images, the 
edges are easily influenced by the pixels not in the 
neighbourhood of roof patches. Many building edges with week 
contrast are missed by NMS of the whole image as shown in 
Figure 8. It can be sure that there are building edges in the 
buffer area. So building edges can be more efficiently derived 
by NMS in buffer area, which provide more edges to 
compensate the LIDAR data as shown in Figure 8. 
Figure 8. Comparison of NMS. The left are edges around roof 
patches by NMS of the whole image, and the right are edges 
derived by NMS in buffer area. 
The overlay of roof patches and edges derived by NMS in 
buffer area is shown in Figure 9. There are some hollows in 
patches because the density of point cloud does not match the 
image resolution. The patches and edges are integrated into new 
patches by morphological closing operation as shown in Figure 
10. Moreover, the hollows in patches are fitted by the closing 
operation, which is beneficial to edge extraction from the new 
patches. 
Figure 10. Integrating of patches and edges by morphological 
closing operation 
The ultimate edges are determined by the edge extraction 
method of mathematical morphology. The edges are closed and 
thin with one-pixel width. Not only the boundaries but also the 
edges of different building layers are extracted completely.