Full text: XVIIIth Congress (Part B2)

  
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EASTING (m) 
Figure 3. Differences in Y (Northing) between the 
WisDOT and Leica AT values, as a function of Easting 
along the strip. 
3.2 Surface Generation 
Two DEMs were generated using the DPW 770 with a 
grid spacing of 5 meters and 2 meters, respectively. 
Only the 2 meter DEM was compared to the WisDOT 
DTM file MIL23X. The 23,799 X, Y, Z values in the 
MIL23X were treated as check points. Elevations from 
the softcopy generated DEM were then interpolated us- 
ing a bilinear interpolation at each DTM point, and then 
compared to its corresponding value in MIL23X. The 
differences are summarized in the first column of Table 
2. Many of the points in MIL23X were derived from 
break lines. 
The second column in Table 2 presents statistics of the 
comparison between the Leica DEM and the 4,405 
points in MIL23X that were not derived from break 
lines (points marked *REG"). The "Difference" in this 
table refers to the value of WisDOT elevations minus the 
Leica elevations at the same spatial location. 
Table 2. Comparison Statistics Leica's DEM and Wis- 
DOT's DTM. 
  
  
  
  
  
  
  
Elevation Points within WisDOT DTM 
All Points “REG” Points Only 
Ave. Difference (m) -0.08 -0.10 
Variance (m) 0.57 0.51 
S.D. (m) 0.75 0.72 
RMS (m) 0.75 0.72 
Number of Points 23,799 4,405 
  
  
  
  
  
In order to investigate the large root mean square (RMS) 
differences that are evident in Table 2, a histogram of 
the number of points with a given difference was calcu- 
lated. The histogram, shown in Figure 5, displays the 
frequency of points with a given difference versus the 
differences. 
342 
  
Frequency 
  
  
  
Difference in Elevation (m) 
Figure 4. The frequency of points within the WisDOT 
DTM with a given difference in elevation between the 
WisDOT DTM and the Leica DEM. 
The histogram shows that the differences are nearly 
equally distributed on either side of the mean difference. 
This may indicate that the models are tipped relative to 
each other. To further investigate, the Leica DEM and 
the interpolated DEM, that is derived from the WisDOT 
DTM, are visualized in Figures 5 and 6, respectively. 
The differences between these two DEMs are not evident 
from these figures. 
In order to determine which of the DEMs is most likely 
correct, a further comparison was made between the 
elevations derived from the WisDOT AT values and the 
elevations from the two DEMs. Fifteen WisDOT AT 
elevation points (from the pugs) were found within each 
of the DEMs. Two of the points (73 and 78) were lo- 
cated toward the edge of a photograph. Table 3 shows 
statistics derived from comparing elevations within the 
Leica DEM to the WisDOT AT derived elevations. The 
first column presents the results from comparing all 15 
WisDOT AT elevation values to the corresponding ele- 
vations derived from the Leica DEM. The second col- 
umn contains the same comparison for the 13 WisDOT 
AT points away from the photo edges. Column 3 pres- 
ents statistics of comparing the 15 Leica derived AT 
elevations to the corresponding elevations derived from 
the Leica DEM. Column 4 contains the same compari- 
son in column 3, excluding points 73 and 78. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996
	        
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