International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004 
  
  
  
  
  
  
  
  
generally very reliable even in wooded areas but are frequently 
missing or erroneous in the river valley and adjoining quarries, 
both of which are open areas. 
Both DEDSs are recalculated to a regular grid with 20m 
posting. and the differences are analyzed. 
  
^ 
  
| -9 | 
-8 | 
E44 | 
-5 1 
o 4 | 
o 3 | 
c 2 | 
o A | 
5 © | 
1 | 
= 2 | 
© 3 | 
- 4 | 
+ 5 | 
ES 5 | 
o 7 | 
8 | 
L 9 | 
10 | 
»10 | 
0 500 1000 1500| 
# grid points | 
  
Figure 3. Test area 1. Z-value of DSM minus z-value of DTM based on 
contour lines. 
For area 1, figure 3 shows a big deviation. The interesting 
values here are the differences with absolute values larger than 
10m. In both cases they indicate situations where one of the two 
DEDSs is clearly not reflecting the terrain surface. Negative 
values indicate areas where the DTM based on contour lines is 
higher than the DSM. This is the case in quarries where contour 
lines where falsely interpolated and to a certain extent it reflects 
the lack of contour lines in the river valley which leads to a 
DTM somewhat floating above the valley bottom. Positive 
/alues indicate areas where the DSM yields values higher than 
the DTM, generally indicating wooded areas. 
  
< 
-9 
— -8 
= =f 
2 E 
e. 3 
9 7 
o 0 
1 
E 
3 
ad 4 
L 5 
Ds 
o 7 
8 
+ 9 
10 
>10 
  
0 2000 4000 6000 8000 
# grid points 
  
  
  
Figure 4. Test area 2. Z-value of DSM minus z-value of DTM based on 
contour lines. 
For area 2 the average difference and variation are smaller than 
in area 1. There are no pronounced peaks above the absolute 
536 
value of 10m. Both DEDSs here result in roughly the same 
DTM . 
The differences can also be stored in a rastermap for analysis of 
the relation between differences and the topographical situation, 
as depicted in the following figure. 
t 
  
Figure 5. area 1: Differences in height between DTM based on digitized 
contours and DSM for area |. Red to yellow = DTM higher than DSM. 
Green = DTM lower than DSM. 
  
Figure 6. Orthophoto showing area |. 
The two datasets are combined into a third grid by taking for 
each gridpoint the lowest available value. This results in à 
genetically hybrid DTM which reflects the terrain better than 
either parent grid separately. In quarries and the river valley the 
DSM-derived grid points reflect much better the actual terrain 
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