The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
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the FastStatic method. The resulting horizontal accuracy of 0.10 
m and the vertical accuracy was found at 0.15 m. Each GCP 
was well documented with photographs and drawings to 
provide additional data for C-SAP programme members. 
Additional field survey: During the field survey 3 elevation 
profiles were measured using GPS RTK method. Each profile 
was 3.5 km long with reference points every 20 m. 
3. SOFTWARE USED 
The first phase included: the stereopair orientation, generation 
of a DSM and orthorectification. All image processing for TS-9 
was conducted with Leica Photogrammetry Suite (LPS) 9.0 
software. The results and first insights were presented in 
September 2006 during the ISPRS symposium held in Goa, 
India (Dabrowski et. al 2006). 
In 2008 some calculations were repeated with the newer version 
of the LPS software (v.9.2 Service Pack 1). Both software 
versions used the RPC method. According to ERDAS, Inc. (the 
manufacturer of LPS), LPS 9.2 SP1 introduced changes in the 
triangulation algorithm for the RPC method as well as sensor 
model enhancements for Cartosat-1. To investigate the 
differences between the results from the two versions of LPS 
software, some calculations done originally with LPS 9.0 were 
repeated using LPS 9.2 SP1. 
4. ORIENTATION 
The orientation of the stereopair was established using 4 sets of 
GCPs: 4, 9, 16 and 25, spread evenly across the stereopair (Fig. 
2). The GCPs left out of the sets were selected as independent 
Check Points. The RMSEs (Root Mean Square Errors) obtained 
during triangulation for the Check Points were used as the 
measure of accuracy of the orientation. Table 1 includes detailed 
results. 
# of GCPs 
RMSEX 
RMSE Y 
RMSE Z 
4 
1,49 
1,32 
1,48 
9 
1,46 
1,22 
1,16 
16 
1,38 
1,18 
1,11 
25 
1,21 
1,22 
1,08 
Table 1. The RMSEs calculated for independent Check Points. 
Clearly, each set provides sub-pixel accuracy of scene 
orientation. Similar results were obtained with the same data for 
TS-9 using different software tools (Jacobson 2006, Titarov 
2007). The results suggest 4 to 9 well defined GCPs for scene 
orientation were enough for imagery from the Cartosat-1 
satellite. In the following sections the variant with 9 GCPs is 
referenced 
5. SETTING UP THE STEREO MODEL 
After the orientation phase, the accuracy of the reconstruction 
of the stereo model was tested. The independent test was based 
on manual elevation measurements made at Check Points. The 
results are summarized in Table 2. 
Figure 3. The GCP and Check Point distribution in each 4 sets 
(GCPs = triangles). 
Point ID 
H GPS 
H Model 
dH [ml 
4 
163,55 
163,07 
0,48 
8 
162,63 
163,56 
-0,93 
10 
165,79 
165,10 
0,69 
14 
138,05 
138,30 
-0,25 
16 
169,65 
171,23 
-1,58 
26 
163,82 
165,20 
-1,38 
Table 2. The differences in elevation at GCPs between the 
stereo model and the field survey. 
The small differences between elevation values obtained from 
the satellite imagery and the ones recorded during the field 
survey clearly imply the stereo model’s accuracy as well as the 
eligibility of the model for DSM extraction. The following 
sections analyze quality assessment of the DSM in more detail. 
6. DSM EXTRACTION 
The next phase of the experiment was DSM extraction using the 
autocorrelation method and assessing the resulting model for 
accuracy. The DSM was based on the scene set with 9 GCPs 
which had been found optimal in the previous phase. The DSM 
was calculated with LPS 9.2 SP1, using the exact same 
configuration as in 2006 with LPS 9.0 (Search Area: 21x3, 
Correlation Window 7x7, Correlation Threshold 0.85) 
(Dabrowski 2006/1). 
The results from LPS 9.0 (lesser than LPS 9.2 SP1) were found 
incorrect to a large degree as far as elevation was concerned. 
The discrepancies were in hundreds of meters and covered 
relatively large area. After visually inspecting both images of 
the stereopair two types of errors were found, both of which 
were independent of the satellite sensor, but rather resulting 
from software. 
The first type of errors resulted from the presence of clouds, or 
rather their shadows on the ground as captured on the stereopair.