1371 
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
particular emphasis for the second. With respect to the former, 
the accuracy of the DTMs were also verified for geocoded 
rDTMs (geocoded after the generation process), while for the 
latter it was deeply studied the influence of the GCPs (number 
and the geometric distribution) on the DTM’s accuracy. 
Parameter 
Value 
Number of tie points 
60 
Search window size 
800 pixel 
Moving window size 
30 pixel 
Correlation coefficient 
>0.8 
Terrain detail 
level 6 (over 7 levels) 
Table 1. Optimization of the DTM’s automatic extraction 
procedure. 
43.90000 
43.85000 
43.80000 
■§ 43.75000 
■ 
« 43.70000 
I 43.65000 
3 43 60000 
43.55000 
4.80000 5.00000 5.20000 
Decimal Longitude 
5.40000 
Figure 2. Ground control points distribution over the study area. 
One of the major factor determining the accuracy of the 
generated DTM, as well as the processing time needed for its 
generation, is the ‘terrain detail level’ parameter. Since the 
DTM extraction is carried out by means of automatic image 
matching to find homologous features on the Aft and Fore 
images of the stereo pair, the terrain detail level determines the 
number of image pyramids used during the image matching. 
The use of the ‘minimum terrain detail level’ stops the process 
after the coarsest level of the image matching, while the use of 
the ‘maximum terrain detail level’ iterates until the image 
matching is performed at the highest resolution as possible. 
In this study it was observed that the use of terrain detail level 6, 
instead of the maximum level (level 7), reduced the computing 
time by a factor of 4.6 without any sensible decrease in the final 
DTM’s accuracy. 
3.3 Ground control points selection 
Because the satellite’s pointing and ephemeris information are 
often inadequate for their use in applications involving high- 
resolution imagery, DTMs must be referenced to a map 
coordinate system using GCPs (Lang, 1999). The use of GCPs 
in the DTM generation process brings to the so called ‘absolute 
DTMs’, where ‘absolute’ means that the terrain’s elevation 
values are referred to a geodetic datum. 
When no GCPs are available it is still possible to produce 
DTMs from stereo pairs .by means of automatic image matching 
algorithms,. In this case the so called ‘relative DTMs’ will be 
produced, where ‘relative’ means that the terrain’s elevation 
values are not referred to a geodetic datum but to an arbitrary 
plane (e.g., the lowest value in the scene). 
This study investigated the DTM’s generation for both the 
relative (rDTM) and the absolute (aDTMs) methods, with 
4. RESULTS AND DISCUSSION 
4.1 Medium-resolution DTM generation 
4.1.1 The influence of GCPs in the generation of relative 
DTMs: 
When generating relative DTMs, the output is not correctly 
georeferenced and elevation data are referred to an arbitrary 
plane. Thus, without properly geocoding the error (in the z 
coordinate) of the Cartosat-1 rDTM was observed to be many 
hundreds of meters. After georeferencing the rDTMs using 5 
GCPs of the original C-SAP dataset, the vertical accuracy 
drastically increased to less than 20m (LE90). 
The DTM’s accuracy was tested at two different scales: i) at 
local level using as Independent Check Points (ICPs) the 
remaining 17 GCPs of the original C-SAP dataset, and ii) at 
global scale using the MNTDBTOPO® DTM as ground truth 
for the whole test site. 
For geocoded rDTMs, results showed no significant difference 
in the mean value of residuals (p) for both the comparison to 
the ICPs and the MNT DBTOPO® DTM, respectively 4.33m 
and 4.24m. On the contrary, this is not true for standard 
deviation (a), RMSE and LE90, which showed higher values 
when computed with respect to MNT DBTOPO® DTM 
(<r=13.55m, RMSE=14.19m and LE90=19.00m), indicating that 
the statistics of the ICPs (cj=8.09m, RMSE=9.02m and 
LE90=16.10m) could not be assumed as a correct term of 
comparison for the complete elevation range. 
4.1.2 The influence of GCPs in the generation of absolute 
DTMs: 
When generating absolute DTMs, the number and geometric 
distribution of the GCPs have a great impact in the final DTM’s 
accuracy. For this reason, the original set of 22 GCPs was 
divided into a first subset of j GCPs used for the DTM’s 
georeferencing and a second subset of 22-j ICPs used for the 
evaluation of results. 
Several tests have been done, varying j form 2 to 22 to find the 
optimum number of GCPs to be used. Table 2 shows a 
summary of results. 
With the exception of test #5, the comparison of the Cartosat-1 
aDTM with the ICPs showed a RMSE between 1.41m using 9 
GCPs (for test #14) and 5.17m using 2 GCPs (for test #3), while 
the LE90 was between 1.91m using 6 GCPs (for test #11) and 
8.91m using 2 GCPs (for tests #2 and #3). Even if the observed 
mean values of residuals were small and between -0.12m (for 
test #13) and -2.80m (for test #3), they were not null, thus 
indicating a small bias in the output data. Regarding their 
standard deviation, the observed values were between 1.36m 
(for test #11) and 4.49m (for test #1). 
The best overall performance was obtained for test #14 using 9 
GCPs (p=0.47m, cr=1.39m, RMSE=1.41m and LE90=2.77m),