la, CA, 9-11 Nov. 1999 
ak Line Extraction 
eak lines. Compared with the lines 
othed and connected to longer 
he biased sigma filter. The heights 
ve to be derived from the original 
  
  
  
n the pre-processed slope model. 
ie. break lines extracted from the 
] them with geodetically measured 
out that the whole terrain model 
| in y-direction and 1.2 m in x- 
reason was an insufficient geo- 
points because of lack of suitable 
(d road sides compared to the 
d to determine the shift. After 
the DTM the extracted break lines 
y measured road sides (figure 8, 
| the correct width while the banks 
r this is that the edges of the roads 
n the edges of the banks. Even 
it was difficult to determine the 
pancies lie in the range of 1-2 m, 
nition accuracy of these lines in 
  
  
  
een terrestrially measured and 
ves before and after correcting the 
ference. 
M from both the laser point cloud 
'omorphologically revised digital 
   
International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 3W14, La Jolla, CA, 9-11 Nov. 1999 
3 EXTRACTING BUILDINGS FROM LASER DATA 
The method suggested here has been partly published in (Kraus 
and Rieger, 1999). At first an accurate ground elevation model is 
needed which was done according to (Pfeifer et al., 1999). 
Secondly, a “ground biased” (referred to as “last pulse model") 
and a "crown biased" (referred to as "first pulse model") grid 
elevation models are created. These elevation models are derived 
from the raw laser data by a moving maximum respectively 
minimum filter. Both models show vegetation, yet the amount of 
tree points is much less in the last pulse model. 
The models are called last and first pulse models since they were 
basically derived from summer last pulse and summer first pulse 
laser data, respectively. However, since there is very little 
difference between the two flights (Rieger et al., 1999), it is well 
possible to derive the elevation models from only one flight, even 
from the winter flight. Here, the two models were derived from 
summer first and last pulse, respectively, and used as provided by 
the company “TopoSys” which took the laser flights. 
The grid models were used instead of the raw laser data for 
several reasons: 
" the approach is much easier and is based on standard 
software; 
" there is a necessity to analyse the data in some 
neighbourhood which is difficult to undertake with the huge 
number of laser dots; 
the position of the laser dots is completely arbitrary, and it is 
difficult to find topological neighbours. In a grid there is a 
clearly defined neighbourhood between points; 
the raw laser data must be filtered towards ground 
respectively surface which is done as a-preprocessing step 
through the creation of the grid models. Work on the raw 
laser data would not easily allow to do so. 
  
  
Figure 9: Digital orthophoto of part of the research forest. 
The laser elevation models show absolute elevations which are 
not feasible for the extraction process. In order to obtain "object 
heights" it is necessary to reduce the surface models by the 
ground elevation models which is done by simple subtraction. 
The resulting models show first pulse respectively last pulse 
object heights. 
The following grid models are needed for the extraction process: 
= first pulse — ground; 
= last pulse — ground; 
= first pulse — last pulse. 
  
Figure 10: DEMs from laser data. Grid width 1 m. Left: first pulse — ground; Right: last pulse — ground. 
Linear gray coding for heights: Black means a height value of 0 m, white a value of 40 m. 
189