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Figure 6. Radiometric differences between fore and aft channels. 
  
Figure 8. The triangular meshes of the 10,000 matched points 
used in the 40 m DTM grid interpolation. 
level image showed that some small bumps might have remained 
in the data. The roving GPS data were not available to us, so for 
a quantitative evaluation of the results we did the following. 
Using the DTM an orthoimage with a 13.3 m pixel size from the 
fore channel was generated and the GCPs that were included in it 
were manually measured. Their pixel coordinates were 
transformed to ground coordinates, their height was bilinearly 
interpolated from the underlying DTM, and these values were 
compared to the adjusted ground coordinates of the bundle. The 
RMS difference in X, Y, and Z was 6, 5.7, and 5.1 m, while the 
maximum absolute error was 11.5, 10.5 and 8.1 m respectively. 
Due to the small number of these points (4), the results are not 
conclusive. The second approach was based on orthorectified 
stereo pairs (Baltsavias, 1996). A second orthoimage from the aft 
channel was generated. If the DTM were correct, the 
orthoimages of the fore and aft channels should not have any 
parallax. We measured the parallax with LSM at different well- 
defined points over a regular grid covering the whole image 
format. In addition we subtracted the two orthoimages and 
matched at places where the radiometric differences were high, 
possibly due to remaining parallaxes. All in all, ca. 50 points 
were matched and the maximum parallax was 0.6 pixels, i.e. 8 m. 
115 
  
Figure 9. The second 40 m DTM grid displayed as a grey level 
image. 
A visual control of the difference image showed that most of the 
differences were due to radiometric differences in the original 
images and not due to parallax. Only along creeks some 
parallaxes existed, partly also due to terrain smoothing because 
of the large patch size. 
Some of DTM errors are due to interpolation. The original 
measurements were ca. 10,000 while the DTM had ca. 152,000 
nodes. These errors occur especially in the areas with little or no 
texture (see the triangular meshes for the DTM interpolation in 
Figure 8 and compare to image texture in Figure 1, e.g. at the top 
right part). To reduce this problem we made a second selection of 
points with the interest operator and relaxed criteria. Thus, 80% 
more points were matched and a second 40 m grid DTM was 
interpolated (see Figure 9). A comparison of the two DTMs 
showed an average difference of 0 m, an RMS of 4.2 m, and a 
maximum absolute difference of 26 m. 96.296 of the points had a 
difference of less than 10 m, and 99.9% a difference of less than 
20 m. These measures are an indication of the internal precision 
and repeatability of the algorithm. The maximum difference in 
particular is a good indicator of the maximum error in any of the 
two DTMs. The second DTM, as expected, was modelling better 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996