ISPRS Commission III, Vol.34, Part 3A »Photogrammetric Computer Vision“, Graz, 2002 
  
In summary, the results show that the segmentation algorithm 
tends to merge single tree crowns, which leads to an underesti- 
mation of the number of trees. 
  
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Figure 5b: Area 7, crown closure 89 95, section of 40x40 m: left 
— segmentation result and reference trees, right — tree height 
image and segmentation result (white) 
4.2.2 Derivation of inventory parameters 
The following inventory parameters can be derived from the 
results obtained in Chapter 4.2.1: 
- Crown area 
- Number of trees 
- Tree height 
- Breast height diameter 
- Timber volume 
Derivation of crown area 
The crown area of single trees is one of the most important 
outputs from the segmentation step. The segmentation algo- 
rithm produces a raster file where pixels from the same tree 
crown are indicated by the same label. Crown area is calculated 
by adding up pixels with the same label. Verification of the 
crown area assessment was performed with the 78 correctly 
segmented spruces out of the 197 verification trees in Hohen- 
tauern. The crown areas of these spruce trees were measured by 
terrestrial means. The tree-wise comparison of the crown area of 
the correctly segmented trees and the crown area measured in 
the field showed that segmentation overestimates the crown area 
by 5 m? on average. However, measuring crown area in the field 
is not easy in steep terrain, thus resulting in an uncertainty in the 
verification data. Figure 6 presents the verification results. 
  
  
  
  
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crown area verification trees 
Figure 6: Crown area from segmentation versus crown area of 
verification trees (78 spruces, Hohentauern test site, crown area 
in m?) 
Derivation of number of trees 
The estimation of the number of trees is another output from the 
segmentation process. Figure 7 depicts the verification results 
concerning the number of trees for 23 stands selected represen- 
tatively for the Hohentauern test site (each point in figure 7 
represents on stand). 
  
  
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number of trees verification 
  
Figure 7: Verification in terms of number of trees (23 stands, 
Hohentauern) 
The verification has shown that the number of trees is underes- 
timated. On average the segmentation algorithm captured only 
56 % of the trees within one stand with a standard deviation of 
15 %. However, an uncertainty in the verification data must be 
considered, as the number of trees was not measured in the field 
but estimated from yield tables. 
Derivation of tree height 
After segmentation tree height is derived from laser scanner 
data by searching for the maxima within one crown segment. 
Figure 8 depicts the verification results of tree height measure- 
ments for the 78 correctly segmented spruce trees at Hohentau- 
ern (see verification crown area). 
  
  
  
  
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tree height verification 
Figure 8: Tree height verification versus tree height from laser 
scanner data (78 spruce trees, Hohentauern test site, tree height 
in m) 
A - 307