The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
At first glance the images were cloudless. This impression 
resulted from the low contrast between the shadow-projecting 
clouds and the snow-covered ground. The older version of LPS 
software apparently did not manage to filter excess values. 
The other type of errors which came to light with the LPS 9.0 
was inaccurate elevation extracted along linear objects oriented 
along the left and right edges of the stereopair i.e. parallel (+/- 5 
degrees) to the satellite’s orbit (Fig. 3). 
Figure 3. Gross errors identification using Blend Tool in 
ERDAS Imagine 
Both types of errors identified during the experiment resulted 
not from the sensor mounted on the satellite, but rather from the 
inaccurate sensor modeling by software or its correlation 
algorithm improperly designed. Additional confirmation to 
these claims is the non-existence of the said errors in the newer 
version of LPS (v. 9.2 SP1 or later) based on the same source 
data. The new model significantly differs from the previous 
ones (Fig. 4). 
The differences between models generated by two different 
versions of LPS are obvious. With LPS 9.0 areas with gross 
errors are apparent while the LPS 9.2 SP1 image is much more 
smooth, representing better accuracy of the elevation model. 
Correlation analysis of both the old and the new models with 
the original stereopair shows major improvement (coefficient of 
correlation at 0.85). The LPS 9.2 SP1 image did not have a 
single object in the suspicious class which is coherent with the 
software’s manufacturer statement, regarding the improved 
correlation algorithm for Cartosat-1 introduced with LPS 9.2 
SP1. 
General Mass Point Quality 
% 
Excellent % (1-0.85) 
87.2 
Good % (0.85-0.70) 
12.5 
Fair % (0.70-0.5) 
0.3 
Isolated % 
0.0 
Suspicious % 
0.0 
Table 2. Correlation quality 
7. DSM ACCURACY ASSESMENT 
Accuracy of the DSM generated from Cartosat-1 imagery was 
measured against the two DEMs. Each model was obtained 
independently of the other. After the initial analysis of the DSM 
its accuracy was tested against the reference model based on 
topographic maps with the scale of 1: 25000. The difference 
between the DSM based on the LPS 9.2 SP1 and the reference 
model was very narrow. The graph below illustrates the 
distribution of the differences, most of which tended to stay 
within the +/- 3 m range. That fact proved the high fidelity of 
the obtained elevation models. 
Figure 5. The reference DEM and the calculated DSM 
difference distribution. 
The next phase was aimed at verifying the calculated model’s 
accuracy at the 6 previously chosen Check Points. The results of 
the verification are in the Table 3. Only one of the Check Points 
shows noticeable deviation from the others with the value of the 
difference above 2 m. In all other cases the differences stay 
within the 2 m range, which is further evidence of the high 
fidelity of the model. 
Figure 4. DSMs calculated in LPS 9.0 (left) and LPS 9.2 SP1.