The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
1346 
Quality 
% of vertices 
Excellent 
32 
Good 
37 
Fair 
15 
Isolated 
11 
Suspicious 
5 
Table 6: Quality Report of DEM 
4.2 DEM assessment 
As can be seen in Table 7 and Figure 4, the DEM gives good 
performance (Standard Deviation [SD] 3.83m) for the low altitude 
area category. The SD at GCP locations is around 4.55m. As the 
terrain altitude increases, the SD is also increasing. The results 
degrade with increased altitude. However, this is expected while 
using automatic DEM generation techniques. The SD value is 
around 7m when compared with reference DEM. As the DEM 
generated in the present study is surface DEM, the high values of 
SD are encountered compared to reference DEM, i.e. bare earth 
DEM. 
The DEM analysis is carried out at two modes, i.e. point mode 
and surface mode. At point mode, accuracy is tested at GCP 
locations (old and new provided by PI), random locations with 
different terrain conditions. Surface mode comparison is made 
with reference to SRTM DEM and reference DEM provided by PI. 
The assessment is carried out using the “raster to vector” 
approach, using bilinear interpolation to determine the elevation 
for each CartoSAT-1 DEM 1 Ox 10m cell position in the reference 
data set, by interpolating from the nearest 4 vertices of the 2x2m 
grid. The standard deviation and mean values of the elevation 
differences (between the CartoSAT-1 DEM point and the 
corresponding interpolated vertex) are calculated and stored for 
analysis. The overall results of the comparison with the higher- 
grade reference DEM is presented in Table 7 and Figure 4. 
Description 
Mean (m) 
Std. Dev (m) 
Old GCP Locations 
-3.133 
5.235 
New GCP Locations 
-0.718 
4.546 
Random Locations 
-2.27 
6.86 
Less than 100m 
3.01 
3.83 
100-300 m 
-2.30 
6.25 
More than 300m 
-7.05 
7.33 
Wrt SRTM DEM 
-0.350 
7.047 
Wrt Reference DEM 
-3.259 
7.473 
Table 7. DEM Accuracy Assessment 
The mean values for all the three categories of altitude are ranging 
from -7.05m to 3.01m with respect to the Standard Deviation 
results recorded. The mean value when compared with SRTM 
DEM is not significant, i.e. around zero, as it indicates both are 
surface DEMs. When compared with reference DEM, which is 
bare earth DEM, the difference is around -3.259m. Hence the 
DEMs generated with Cartosat-1 are meeting the specified 
accuracies. These DEMs are extremely useful for applications of 
watershed management. The difference image of CartoSAT-1 
DEM and reference DEM is illustrated in Figure 5. 
0 
8070966 
8H— 
1 
15823971 
2 
3 
15164332 
14148240 
— 
4 
13139328 
—BBB 
5 
12206929 
■Hi 
6 
11115134 
7 
9667716 
8 
8002272 
9 
6380709 
H— 
10 
4901222 
0 
I 
Figure 5. DEM Difference Image along with legend 
Contours have been generated at 10m interval and overlaid on the 
reference DEM which shows clear match (Figure 6) and also 
compared with contours generated from reference DEM.