ISPRS Commission III, Vol.34, Part 3A ,,Photogrammetric Computer Vision“, Graz, 2002 
  
The comparison has shown that the tree height calculated from 
laser scanner data is on average only 62 cm smaller than the tree 
height obtained by ground measurements. 
Derivation of breast height diameter by empirical models 
The breast height diameter (BHD) is one of the most important 
parameters in forest inventories. BHD is a direct function of the 
parameters tree height and crown diameter, which can both be 
measured by means of laser scanning data. An empirical BHD 
model for spruce was developed on the basis of field measure- 
ments of tree height and crown diameter at the Hohentauern test 
site. The calculated BHD model is shown below: 
BHD (mm) 
(h) tree height ( in dm) 
(c) crown area (in m?) 
Spruce (1), Hohentauern, Alpine character: (1) 
BHD --3196-4- 1,33 *5 4 5,19 *c 
R? = 72 %, 165 trees 
Based on this model BHD values can be calculated for each of 
the segmented trees. The statistical analysis of the BHD model 
for spruce shows that tree height (h) accounts for 2/3 and crown 
area (c) only for 1/3 of the BHD model, indicating that accurate 
height data are more important than accurate crown area data. 
This is important since tree height measurements by laser scan- 
ner data are very accurate, whereas crown segmentation still 
leaves room for improvement in terms of accuracy. 
Derivation of timber volume 
Timber volume (vs) which is a direct function of BHD and tree 
height (h) can be calculated by known increment models from 
forestry (Pollanschuetz, 1974). 
BHD (in em) 
h (tree height in m) 
(form value) constant value for different management pro- 
cedures 
vs = (BHD / 1007 * 3,141593 / 4 * h * form value (2) 
The timber volume derived from laser scanner data is calculated 
by using equations (1) and (2) described above. The timber 
volume assessment was tested for the 197 verification trees in 
the Hohentauern test area. Figure 9 presents the correspondence 
of timber volumes measured in the field and timber volume 
calculated on the base of laser scanner data separately for each 
of the 197 trees. 
Figure 9 shows that for some trees the values derived from both 
measurements (laser and terrestrial) do not correspond. How- 
ever, when summing up the timber volumes of the single trees, 
the timber volume derived from laser scanner data is only 
slightly below the actual value (3.3 % of verification timber 
volume). In this comparison all trees were considered, even if 
they had been merged in the segmentation process. Thus, the 
result indicates that the underestimation of the number of trees 
by the segmentation algorithm is compensated by an overesti- 
mation effect due to the merging of various trees into one big 
tree, which results in an overestimation of timber volume for 
this “one” big tree. 
8 
  
  
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vs laser JR model (m?) 
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100 9 : 0? 
0000 #- = 
0000 1,000 2,000 3,000 4,000 5,000 6,000 7,000 &000 
Vs ground truth (n1) 
Figure 9: Timber volume verification trees versus timber vol- 
ume derived from laser scanner data. 
In a further investigation the dependence of timber volume 
accuracy on merging effects was analyzed on the basis of the 
197 verification trees. This investigation has shown that an 
increase of the number of merged trees is associated with a 
decrease of timber volume derived from laser scanner data (see 
Figure 10). 
  
  
  
  
  
  
  
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a Bos 
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2 3061 
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number of trees within one crown segment 
Figure 10: Dependence of timber volume assessment on cor- 
rectly segmented crowns (3 means 3 or more trees) 
S. SUMMARY AND CONCLUSION 
The results of the study show that laser scanning is a potential 
valuable tool in forest inventories. Both the tree-wise approach 
and the statistical stand-wise approach produced reliable results. 
For instance, the timber volumes derived from laser scanner 
data deviated from the timber volumes measured in the field by 
less than 10%. It must be noted that the segmentation methods 
for the tree-wise approach were developed exclusively for 
spruce stands. It must be expected that the methods developed 
within the scope of this project are not suitable for deciduous 
tree species which are characterized by a more complex canopy 
structure. The statistical stand-wise approach should thus be 
chosen for deciduous tree stands. Another restriction is that the 
algorithms have been developed on the basis of a limited forest 
region. Further investigations will be required to verify whether 
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