XXIX-B8, 2012 
VOXEL-BASED APPROACH FOR ESTIMATING URBAN TREE VOLUME FROM 
TERRESTRIAL LASER SCANNING DATA 
C. Vonderach *, T. Voegtle 5, P. Adler? 
* Forest Research Institute Baden-Wuerttemberg, Wonnhaldestr. 4, 79100 Freiburg, Germany — 
(christian.vonderach, petra.adler)@forst.bwl.de 
® Institute of Photogrammetry and Remote Sensing (IPF), Karlsruhe Institute of Technology (KIT), Germany — 
thomas.voegtle@kit.edu 
Commission VIII, WG VIII/7 
KEY WORDS: Terrestrial Laser Scanning (TLS), Forestry, High Resolution Data Acquisition, Automation, Data Modelling 
ABSTRACT: 
The importance of single trees and the determination of related parameters has been recognized in recent years, e.g. for forest 
inventories or management. For urban areas an increasing interest in the data acquisition of trees can be observed concerning aspects 
like urban climate, CO; balance, and environmental protection. Urban trees differ significantly from natural systems with regard to 
the site conditions (e.g. technogenic soils, contaminants, lower groundwater level, regular disturbance), climate (increased 
temperature, reduced humidity) and species composition and arrangement (habitus and health status) and therefore allometric 
relations cannot be transferred from natural sites to urban areas. To overcome this problem an extended approach was developed for 
a fast and non-destructive extraction of branch volume, DBH (diameter at breast height) and height of single trees from point clouds 
of terrestrial laser scanning (TLS). For data acquisition, the trees were scanned with highest scan resolution from several (up to five) 
positions located around the tree. The resulting point clouds (20 to 60 million points) are analysed with an algorithm based on voxel 
(volume elements) structure, leading to an appropriate data reduction. In a first step, two kinds of noise reduction are carried out: the 
elimination of isolated voxels as well as voxels with marginal point density. To obtain correct volume estimates, the voxels inside the 
stem and branches (interior voxels) where voxels contain no laser points must be regarded. For this filling process, an easy and 
robust approach was developed based on a layer-wise (horizontal layers of the voxel structure) intersection of four orthogonal 
viewing directions. However, this procedure also generates several erroneous “phantom” voxels, which have to be eliminated. For 
this purpose the previous approach was extended by a special region growing algorithm. In a final step the volume is determined 
layer-wise based on the extracted branch areas 4; of this horizontal cross-section multiplied by the thickness of the voxel layer. A 
significant improvement of this method could be obtained by a reasonable determination of the threshold for excluding sparsely 
filled voxels for noise reduction which can be defined based on the function change of filled voxels. Field measurements were used 
to validate this method. For a quality assessment nine deciduous trees were selected for control and were scanned before felling and 
weighing. The results are in good accordance to the control trees within a range of only -5.1% to +14.3%. The determined DBH 
values show only minor deviations, while the heights of trees are systematically underestimated, mainly due to field measurements. 
Possible error sources including gaps in surface voxels, influence of thin twigs and others are discussed in detail and several 
improvements of this approach are suggested. The advantages of the algorithm are the robustness and simple structure as well as the 
quality of the results obtained. The drawbacks are the high effort both in data acquisition and analysis, even if a remarkable data 
reduction can be obtained by the voxel structure. 
1. INTRODUCTION be applied to the urban environment. New allometric relations 
must be developed for a rapid and easy measurement of urban 
  
Until recently, forest stands have been the main focus 
concerning national carbon inventories and conservation, but 
recently the importance of urban trees is being increasingly 
Investigated. Urban habitats differ significantly from natural 
systems (Norra, 2009) due to site conditions (e.g. contaminants, 
regular disturbance, climate, and species composition). In 
contrast to forest stands urban trees are planted for ecological 
and social services for the local population (Konijnendijk et al., 
2005). Therefore, the urban tree species composition is 
dependent on site conditions, the arrangement of recreational 
areas and (gardeners) experience and taste. Consequentially, 
urban trees differ in structure from those grown in forest stands, 
and allometric relations developed for forest situations cannot 
tree biomass (Zianis, Mencuccini, 2004). For this purpose an 
extended approach was developed for a fast and non-destructive 
determination of the branch volume of a tree, DBH (diameter at 
breast height) and tree height in urban environment from point 
clouds of terrestrial laser scanning (TLS). In the next chapter, 
related literature is compiled and commented, followed by a 
section about the used test material, data acquisition and pre- 
processing. In chapter 4, the data analysis is described in detail; 
results of practical application including a quality assessment 
are shown in chapter 5, while conclusions and an outlook are 
given in chapter 6.