ISPRS Commission III, Vol.34, Part 3A „Photogrammetric Computer Vision", Graz, 2002 
  
Forest floor model 
Because of difficult terrain conditions at the test sites (steep 
slopes and small structured relief) considerable attention was 
placed on the question of generating accurate forest floor 
DEMs. Raw last pulse data were processed in a regular 1 x 1m 
grid using a multi-resolution method in combination with an 
improvement by the integration of gradients. 
The method searches the raw data for the minimum values 
within the respective resolution and increases the minimum by 
an appointed amount as a function of the gradient in the 
neighborhood. Smoothing and thresholding algorithms are 
performed in each resolution step, thus resulting in correspond- 
ing pixel values. The method starts with a coarse resolution (e.g. 
10 m) and compares the results with better resolutions (e.g. 7, 5, 
3, 1 m) step by step. Hence, the method allows forest floor 
information to be obtained even in dense stands, as it is as- 
sumed that within 10 x 10 m at least one laser signal comes 
from the ground. The multi-resolution method - decisions 
whether to take the coarse or better resolution are made by 
thresholding - assures that ground results are taken from better 
resolutions whenever there is a ground signal, in other cases 
(dense stands) the value from the coarse resolution is taken. The 
accuracy obtained varies between 18 and 45 cm. More detailed 
information on the filter method is given in Ruppert et al., 2000. 
3. TEST SITES AND DATA USED 
Test sites 
Investigations were carried out at two Austrian test sites: 
-  Hohentauern test site: 
mountainous (Alpine) test site characterized by high relief 
energy and steep slopes (ranging from 1200 - 1700 m); 
tree species mainly consist of spruce (94 96) 
- Eastern Styria test site (Burgau and Ilz): 
small structured hilly test site located at the south-eastern 
border of the Alps, which is characterized by moderate 
height differences (ranging from 250 - 400 m) and small 
troughs and hillocks; the main tree species are beech (30 
%), spruce (30 %) and pine (15 96). 
  
Laser scanner data 
The described approaches are only feasible if the following 
parameters are fulfilled: 
- high measurement density in order to achieve a good 
separation between individual trees 
- steep viewing angle in order to have a sufficient number 
of ground points (Hyyppa et al., 1999, Samberg et al., 
1999) 
- two different modes - first pulse and last pulse - to obtain 
ground information (last pulse) on the one hand and in- 
formation of the crown surface on the other hand. 
The laser scanner campaigns were therefore carried out using 
the TopoSys I scanner because of its high measurement density, 
steep viewing angle and capability of providing both first and 
last pulse modes. 
The following laser scanner data were acquired for the two test 
sites: 
-  Hohentauern test site: 
First pulse: 22.8.1999; flight height 800m; 4-5 points per 
2 
m 
Eastern Styria test site: 
First pulse: 23.8.1999; flight height 800m; 4-5 points per 
2 
m 
Last pulse: 26.3.1999; flight height 800m; 4-5 points per 
2 
m 
4. METHODS AND RESULTS 
This chapter describes the methods and algorithms used for the 
different processing steps necessary to implement the ap- 
proaches outlined in Chapter 2. 
4.1 Method and Results of the Stand-wise approach 
The stand-wise approach was carried out for the Hohentauern 
and Ilz test sites. Data processing is based on first pulse data 
(crown DEM) which are resampled to a regular 1 x 1 m grid 
using a simple algorithm, which searches for maximum values 
within the grid and the forest floor model roughly described in 
Chapter 2. 
4.1.1 Assessment of top heights 
The aim of the study was to assess the top height on a stand- 
wise basis. The top height is an important inventory parameter 
and can be defined as the average height of 2096 of the strongest 
trees of a stand. The processing steps and the statistical analyses 
are described in the following sections. 
In a first processing step a maximum filter (window size 3 x 3) 
is applied to the first pulse data (tree height image) in order to 
eliminate pulses which are reflected on the lower parts of the 
trees. Based on the filtered laser data (max laser) the following 
parameters were extracted at stand level. 
- arithmetic mean of the laser derived tree heights (h), 
stand-wise 
- crown closure, assuming heights larger than 6 m to be 
crown hits (c), 
For this purpose the digital stand boundaries obtained from the 
forest owners were superimposed on the laser scanner data. 
The arithmetic means (h) derived stand-wise from the filtered 
tree height image were compared with the top heights provided 
by the forest owners. Table ldepicts the statistics of this com- 
parison for each test site separately and for both test sites to- 
gether. 
  
  
  
  
  
Mean differ- Standard 
Number of eu 
stands ence top deviation of 
heights — h differences 
Hohetauern 139 -0.3256 5.1259 
Ilz 49 -0.9099 5.2412 
both test sites | 188 -0.4949 5.0520 
  
  
  
  
Table 1. Differences of tree heights derived from laser scanning 
data and top heights from the forest inventory 
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