ISPRS Commission II, Vol.34, Part 3A „Photogrammetric Computer Vision‘, Graz, 2002 
  
    
(a) Initial vector (b) After 35 iterations 
(c) Final result 
Experimental results on synthetic images 
The approach was then experimented on Landsat 7 images 
using 7 bands (30m resolution, we ignored band 6) to update 
features of the Canadian National Topographic Database 
(NTDB). The presented result in figure 2 show a homogeneous 
water region. The vector initializing the snake is shown in 
white on image (a) over enhanced band 2 of the Landsat7 
image. Notice the lighter region inside the water area due to 
dense vegetation covering part of the lake and mostly obvious 
on this specific band. The mixture evaluation clearly defined 
two textures for this type of area that initially seems to be 
homogeneous on other bands. Over the enhanced image of 
band 4, an intermediate result of the snake process is presented 
on image (b) and the final result is presented on image (c). The 
specific parameters values for this result are the same as the 
previous example. 
RTE 
  
(a) Initial vector (b) After 70 iterations 
Figure 2. Experimental results for water region on 
Landsat 7 images 
(c) Final result 
Finally, results of the experiment on 7 bands of Landsat 7 
images for textured regions like vegetation are shown in Figure 
3. Notice that is difficult to visually delineate this kind of area 
on any of the bands. We present the result on an enhanced 
image of band 8 (15m resolution). The initial snake is 
presented in image (a), an intermediate image of the snake 
process is shown on image (b) and image (c) presents the final 
result. The only specific parameter different from the previous 
examples is the decreasing factor, À , that was set to 0.7. 
    
(b) After 40 iterations 
Figure 3. Experimental results for vegetation area on 
Landsat 7 images 
(c) Final result 
(a) Initial vector 
7. CONCLUSION 
Our work demonstrates the importance of using snakes and 
multi-spectral images for updating existing spatial area 
information. We propose a promising automatic approach for 
the update of existing vectors in topographic databases. The 
combined statistics and gradient information for the external 
energy allows the snake to grow in both directions and 
provides complementary measures to precisely guide its 
deformation. Indeed, the proposed weighted MAP estimation 
strategically takes advantages of their respective strengths and 
overcomes their drawbacks. An efficient implementation using 
finite elements has been proposed for accurate localization. 
Finally, experimental results demonstrate the reliability of the 
approach. 
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