The three laser flights provide data with absolute (sea level) 
elevation. In contrast the reference tree data are given relative to 
the ground level. Thus the laser elevations must be reduced to the 
ground. This is done by interpolating a high quality ground model 
(DEM) from the laser data (Pfeifer, 1999). The laser elevations 
are then reduced by the DEM elevations at the respective 
positions. Figure 2 shows a perspective view of the laser dots of 
summer first pulse for a mature beech stand reduced to ground. 
Figures 3 to 5 show histograms and cumulative frequencies of the 
International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 3W14, La Jolla, CA, 9-11 Nov. 1999 
   
relative elevations for the three flights, summer first (sf), summer 
last (sl), and winter last (wl), for three selected stands. The height 
interval for the classification (class width) is 1 m. The histograms 
are plotted with (tree) height vertically and relative frequency 
horizontally in order to give a better impression of the close 
connection between tree forms and (relative) number of reflected 
signals. The cumulative frequency curves show some typical 
patterns depending on the tree species: 
e With deciduous trees there are no off-terrain reflections in 
  
50 
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40 | — Summer last 
30 -- Winter last 
20 
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Figure 3 Histograms and cumulative frequencies of the tree heights above ground for the three flights for a mature beech stand. 
Height above ground [m] 
  
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40 | — Summer last 
30 Winter last 
20 
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Figure 4 Histograms and cumulative frequencies of the tree heights above ground for the three flights for a spruce stand. 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
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igure 5$ Histograms and cumulative frequencies of the tree heights above ground for 2 mixed spruce and beech stand. 
104 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
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Internatior 
winter last. 
e Coniferous tree 
summer. 
e For both decid 
slight differenc 
pulse. 
The last observation : 
go to the extra effo 
TopoSys scanner doe: 
signal at a time, so thi 
The penetration rate c 
cumulative frequency 
showed that any per 
deciduous trees in thi 
presence of coniferou 
and coniferous trees i: 
summer flight. Since | 
in the project area, i 
different deciduous or 
The penetration rate si 
pulses. This is not a 
intensity of the refle 
recognized by the las: 
reflected laser dots. ' 
clearly recognized as 
coniferous trees, gene 
than with deciduous tr 
scanner uses an infr 
intensity patterns obsei 
In the winter flight t 
species is much lower 
conditions: The summi 
of hot dry weather, wh 
period. Obviously dry 
reflectors of the laser s: 
  
Figure 6 Number of