The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
1366 
5. DSM GENERATION AND ACCURACY ANALYSIS 
The image matching process was realized by using the program 
package SAT-PP developed at IGP, which has been used 
extensively for various platforms, sensors and image scales with 
very good results, published for several projects. More details 
about the underlying algorithm can be found in Zhang (2005). 
SAT-PP produced very densely matches. It is using multiple 
match-point primitives (regular image grid points, interest 
points and edgels). For both test areas, we chose a modified 
cross-correlation matching and no LSM at the final stage. 
A quantitative 3D evaluation was realized by co-registering the 
generated DSM and the reference DSM/DTM by using the 
method described in Gruen and Akca (2005). With the 
respective program LS3D, three shift parameters (Tx, Ty and 
Tz) were determined between the two dataset to remove 
possible offsets. After this co-registration, the Euclidian 
distances (E) between the two DSMs are computed point-wise, 
and the error is also split into its X, Y and Z components. 
5.1 Testfield Catalonia 
For the whole area, a DTM with a grid size of 15 m was 
provided. For the generated DSM, we chose a grid size of 10 m, 
corresponding to 4 times the GSD of Cartosat-1. About half of 
the testfield is a hilly region with forest, trees and bushes and 
open area. The rest is a flat area with a part of the city of 
Barcelona, some villages, trees and open areas. A lake and the 
sea were excluded from the evaluation (see Figure 4). 
Figure 4. Generated DSM of the testfield Catalonia (grid 
spacing 10m). The black area is an excluded lake area. The size 
of the evaluation area is 29 km x 25 km. 
The results of the 3D evaluation are summarized in Table 5 and 
visualized in Figure 5. The residuals smaller than -3 sigma and 
larger than +3 sigma are very few. The larger errors are mainly 
caused by shadow areas and differences between the reference 
DTM and the matching DSM, like trees (see reddish areas in 
Figure 5) and multitemporal differences, like surface mining 
(see white circles in Figure 5), while the bluish arc at the 
bottom left is due to a highway. 
Figure 6. Blunder area resulting from different viewing angles 
causing large perspective differences at terrain with large slope 
and unfavorable aspect (see also Figure 8). 
Figure 7. Generated DSM of the volcano Sakurajima, grid 
spacing 5 m. The central red/blue part corresponds 
approximately to the DSM evaluation area (4km x 4km). The 
black area is excluded because of clouds in the images 
Figure 5. Color coded image of the Euclidian distances (DSM - 
reference). The color intervals correspond to 5m. The two white 
circles show the blunders caused by surface mining. 
5.2 Testfield Sakurajima 
The main characteristics of the steep volcanic area results in the 
following matching conditions: a large part of the area of 
interest was covered by shadows, clouds obscure a part of the 
image, the texture is generally weak, while the perspective 
differences at steep slopes were very large (see Figure 6).