The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
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are available (stereo imaging direction). A threshold of 0.5 
pixel is used throughout this report for acceptance of forward 
intersection results. 
2.4 Blunder reduction 
Blunder reduction is done during and after image matching and 
during forward intersection. Most of the methods for image 
matching are mentioned in section 3.1. Additionally, some 
blunders can be detected via quasi-epipolar reprojection of the 
stereo pair. For this best points from hierarchical matching are 
used to estimate an affine transformation from fore to aft 
CARTOSAT image. The standard deviation a for the column 
residuals of this affine transformation is very small (a = 0.2-0.3 
pixel). The fore coordinate pairs of the mass stereo tie points 
are transformed with this affine transformation. All points are 
rejected for which the absolute value of the difference of the 
column coordinates of the aft tie points and the transformed 
fore tie points is larger than the threshold 3a. This blunder 
check is not independent of the blunder check via the residuals 
in image space during forward intersection. 
2.5 DSM interpolation and orthoimage generation 
A regular DSM is generated from the point clouds produced by 
forward intersection via triangulation and interpolation (Hoja et 
al. 2005). DSM editing via cloud- and water-masks etc. is under 
investigation, as well as DSM fusion for separately produced 
CARTOSAT-1 DSM and existing DSM from other sources. 
After derivation of the DSM orthoimages with user defined 
datum and projection can be generated using the affine 
corrected RPC and the DSM. Accuracy evaluations based on 
orthoimage comparison of aft and fore sensor are given in 
(Lehner et al. 2007) with more details. 
3. RESULTS FOR THE DIFFERENT TEST SITES 
3.1 Catalonia 
A reference DTM with a GSD of 15 m (height accuracy 1.1m) 
and 10 orthoimages with a resolution of 0.5 m are provided by 
the Institut Cartografic de Catalunya (ICC). 70 GCP have been 
measured in the orthoimages and the stereo partner Cat-A with 
sub-pixel precision. These measurements have been 
automatically transformed into Cat-A/F tie points via least 
squares matching using mass points from hierarchical matching 
for initial guesses of Cat-F coordinates and least squares 
matching (LSM) with several window sizes. Thus, 68 GCP for 
Cat-F could be derived - well fitting to the Cat-A GCP in terms 
of stereo tie points. 6 window sizes from 17 to 27 have been 
used in LSM in order to get statistical values for the accuracy. 
The mean standard deviations in rows and columns for the 68 
GCP and 6 window sizes were below 0.1 pixel. Consistent 
stereo tie points are derived by this procedure. 
The standard deviations of the residuals in RPC correction are 
given in table 3 and a plot of the residual vectors for the fore 
image is provided in figure 2. The shift parts of the affine 
transformations for RPC correction are given in table 4 for all 
stereo pairs, normally (besides M1A/F) corresponding well with 
the expected orbit/attitude accuracy of a few hundred meters 
(better than 160 m along-track and 100 m across-track). 
After RPC correction the DSM is derived through image 
matching and forward intersection. A few numbers for 
illustrating the results are given in table 5. For each stereo pair 
the number of matches passing through forward intersection 
(using the threshold of 0.5 pixel for all 4 residuals in image 
space) is given for the best tie points from hierarchical 
matching (HM) and for all points after region growing (RG). 
Additionally, the mean height difference and the standard 
deviation of the height differences to the reference DSM/DTM 
are provided. There is no distinction on land use. Thus, for the 
small Taching area with a high percentage of forests with tall 
trees this gives a wrong picture which will be put right to some 
extent in the section 3.3 on Bavaria. 
Image 
Number 
of 
GCP 
Shift part of affine 
transformation (pixel) 
row 
column 
Cat-A 
70 
-32.66 
-16.95 
Cat-F 
68 
-48.42 
1.47 
MIA 
31 
-2231.59 
-685.14 
M1F 
30 
-2335.86 
-546.46 
M2A 
9 
-40.08 
-6.01 
M2F 
9 
-50.83 
-0.56 
Bav-AT 
14 
52.38 
-8.60 
Bav-FT 
14 
62.07 
38.18 
Table 4: Shift parts of the affine transformations for RPC 
correction for visualization of absolute positional accuracy of 
original RPC 
Stereo pair / 
matching 
step 
Number of 
accepted tie 
points 
(million) 
Height difference: 
reference DTM/DSM minus 
Cartosat-DSM (m) 
mean 
a 
Cat / HM 
0.06 
-0.6 
1.83 
Cat / RG 
7.08 
-1.0 
3.05 
Ml / HM 
0.01 
-1.9 
2.24 
Ml / RG 
4.82 
-1.4 
3.80 
M2 / HM 
0.03 
-1.5 
2.21 
M2 / RG 
6.14 
-1.1 
3.53 
Bav / HG-T 
0.006 
-3.6 
4.18 
Bav / RG-T 
1.09 
-3.6 
7.29 
Table 5: Number of tie points accepted during forward 
intersection for 2 matching steps (HM: excellent points from 
hierarchical matching / RG: all points from region growing) and 
mean and standard deviation of height differences to the 
reference DSM (Ml/2) or DTM (Cat and Bav, T: Taching area 
only) 
Orthoimages Cat-A-ortho and Cat-F-ortho are computed based 
on corrected RPC and the DSM produced via triangulation and 
interpolation from the accepted matches from region growing. 
The matching between Cat-A/F-ortho gives shift vectors with a 
mean of (0.10, 0.06) and a standard deviation of (0.16, 0.24) for 
rows and columns, respectively (in pixel, about 45000 tie points 
excluding the coast area are used). Thus, in regions of 
acceptable density of matches, i.e. in areas where the DSM 
interpolation has enough support, the fit between the 
orthoimages is very satisfactory.