Uwe Bacher 
  
4.3 Branches in Shadow Projection 
The extraction of branches in shadow projection is geometrically restricted using the hypotheses for the trunks and makes 
use of the properties of the chosen line extraction. The search areas are defined by ellipses with the hypotheses for the 
trunks defining their longer axes. The smaller axes have half the length. As the type of the tree is not yet known, the 
size of the ellipse, has to be chosen relatively large. Within these ellipses distinct dark lines are extracted (cf. Fig. 6a). 
We exploit the knowledge that branches are starting at the trunk and are of decreasing thickness by exploiting hysteresis 
thresholding (Canny, 1986) in the following way: The upper threshold corresponding to the starting points of the tracking 
is set very high to get only lines with high contrast, i.e., the trunk. On the other hand, the lower threshold controling the 
extent of the tracking is chosen relatively low, to track branches to their final ends. Only light smoothing is used in the 
image to detect also very thin lines. As the employed version of the line extractor does not work recursively, at every 
branching only one line is tracked. Figure 6b) shows results for the approach in another image part. Figure 7 presents 
the results for two different hypotheses for trunks in this image part. In one case, the hypothesis represents the trunk of a 
deciduous tree, but in the other case a disturbing object, i.e., a street-lamp a fact which could already be derived from the 
extracted branches. 
    
    
a) Example 1 b) Example 2 
Figure 6: Tracking of branches (Search area = black ellipse; branches = grey) 
  
     
a) Trunk-hypothesis 1 b) Trunk-hypothesis 2 
Figure 7: Branches in example 2 for different trunk-hypotheses 
5 CONCLUSIONS 
With our investigations we have demonstrated that it is feasible to extract leafless deciduous trees from high resolution 
aerial imagery from their shadow and their direct projection in many cases. Even fine branches can be extracted at a 
ground pixel size of about 4 cm. 
The tracking of the branches can be extended to all possible lines. Trees and disturbing objects, such as street-lamps, can 
  
56 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 
  
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