The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
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aDTMs behaved similarly and in near flat terrain they showed a 
high accuracy (RMSE<4.52m and LE<7.00m). With increasing 
the slope the RMSE and LE90 rapidly increased until 
unacceptable values (greater than 22.00m for the RMSE and 
greater than 35.00m for the LE in the range of 30°-40° for 
slopes). It is to be noted that for the Salon de Provence test site 
94.12% of samples had a slope less than 20°, consequently the 
resulting DTM’s accuracy was better than 10.74m for RMSE 
and better than 17.00m for LE90. 
Another term of comparison used for evaluating the DTM’s 
performances were the requirements for IGN’s and Spot 
Image’s Reference 3D®. The Reference 3D® is a DTM 
generated using automatic correlation from SPOT-5/HRS data. 
Its specifications are given in Table 4 (Buillon et al., 2006). 
As shown in Table 5, the Cartosat-1 aDTM fulfilled the 
Reference 3D® requirements while the rDTM did not. 
4.2 Low- resolution DTM generation 
The performances of the Cartosat-1 satellite were also tested for 
the generation of low-resolution DTM (90m grid resolution). 
As term of comparison it were used: 
- The NASA’s SRTM DTM; 
■ The MNTDBTOPO® DTM downsampled to a 90m 
cell resolution; 
■ A set of ICPs extracted from the original C-SAP 
dataset. 
Results show that the 90-meters Cartosat-1 aDTMs performed 
better than the SRTM DTM and overcome the SRTM’s 
specifications, while the 90-meters Cartosat-1 rDTM poorly 
performed and did not met the SRTM’s specifications. 
Regarding the comparison to the higher precision ICPs, the best 
results were obtained for test #14 (p=-0.12m, o=3.16m, 
RMSE=3.09m and LE90=4.09m), followed by test #5 
(p=0.69m, <T=3.32m, RMSE=3.32m and LE90=5.34m) and the 
SRTM performed worse (p=0.51m, cr=3.37m, RMSE=3.33m 
and LE90=7.38m). 
Even if looking at mean values of residuals, standard deviations 
and RMSEs the Cartosat-1 aDTM and the SRTM have similar 
values, the former is more accurate in terms of LE90: that 
means that the Cartosat-1 low-resolution aDTM has fewer gross 
errors than the SRTM. 
Parameter 
Specifications 
DEM resolution 
larc second (~30m on the Equator; 
21m at 45° of latitude) 
Planimetric 
absolute accuracy 
15m at 90% confidence level 
Altimétrie 
absolute accuracy 
10m at 90% confidence level, for 
slopes lower than 20% 
18m at 90% confidence level, for 
slopes included in 20% and 40% 
30m at 90% confidence level, for 
slopes greater than 40% 
Planimetric 
relative accuracy: 
10 m at 90% confidence level 
Altimétrie 
relative accuracy 
5m at 90% confidence level, for slopes 
lower than 20% 
15m at 90% confidence level, for 
slopes included in 20% and 40% 
28m at 90% confidence level, for 
I I slopes greater than 40% | 
Table 4. Reference 3D® specifications. 
When comparing all the low-resolution DTMs to the 
downsampled MNT DBTOPO®, it seems that the SRTM has a 
better accuracy (p=-0.87m, cr=5.92m, RMSE=5.99m and 
LE90=8.00m) than the Cartosat-1 low-resolution aDTMs 
(respectively p=-0.41m, cr=6.47m, RMSE=6.48m, LE90=9.00m 
for test#5 and p=- 1.21m, a=6,82m, RMSE=6.93m and 
LE90= 10.00m for test# 14), but these results may be due to an 
inaccurate estimation of the elevation data in the downsampled 
MNT DBTOPO® DTM. 
As expected, the Cartosat-1 low-resolution rDTMs showed 
inaccurate results (p=4.20m, cr=13.51m, RMSE=14.14m and 
LE90=19.00m). 
When considering the requirements for IGN’s and Spot Image’s 
Reference 3D®, the Cartosat-1 low-resolution DTMs (both 
relative and absolute models) met the requirements only for 
slope less than 20%, while for higher slopes they did not met 
the requirements. 
5. CONCLUSIONS 
For the Salon de Provence test site, it was fully investigated the 
potentialities and limits of the 2.5m Cartosat-1 stereo images 
for generating DTMs using commercial off-the-shelf software, 
so from the point of view of a typical user. 
When generating relative DTMs, if not properly geocoded it 
was observed an error in the elevation values of some hundreds 
of meters. After georeferencing, the RMSE was between 9m 
and 14m and the LE90 between 16m and 19m. 
When generating absolute DTMs, the optimum number of 
GCPs was found to be nine, with a regular geometric 
distribution (i.e., three in the upper part of the image, three in 
the centre of the image and three in the lower part of the image), 
while the minimum number of GCPs needed to get good DTMs 
was found to be three (i.e., one in the centre, one in the lower 
left and another in the lower right). In this cases, the absolute 
DTMs fulfilled the standards required for the IGN’s and Spot 
Image’s Reference 3D® and could be used for deriving such 
product. 
Finally, the comparison of the downsampled Cartosat-1 DTMs 
with the NASA’s SRTM showed that the former fulfilled the 
SRTM’s specifications and could be successfully used to fill 
gaps in SRTM’s DTMs. A similar conclusion was also found 
investigating the Mausanne les Alpilles test site (Gianinetto, 
2008). 
Slope 
range (%) 
Reference 3D® 
requirements (LE90) 
rDTM 
(LE90) 
aDTM 
(LE90) 
0-20 
10.00 
12.00 
6.00 
20-40 
18.00 
36.00 
14.00 
>40 
30.00 
58.00 
27.00 
Table 5. Accuracy of the Cartosat-1 relative and absolute DTMs. 
Statistics on residuals (LE90) computed with respect to 
Reference 3D® requirements