International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004 
  
image acquisition and processing resulted as the only 
alternative to the aerial photos or any other surveying method. 
Nowadays space images are competing successfully with 
traditional aerial photos, for the purpose of DTM generation or 
terrain study in such problematic countries, as Afghanistan is. 
Also the resolution and availability of world-wide scenes taken 
from satellite platforms are constantly increasing. Those scenes 
are available in different radiometric modes (panchromatic, 
multispectral) and also in stereo mode. 
2.1 The satellite images 
The HRG sensor carried on SPOT-S since May 2002 was 
suitable for our tasks [Spot Image]. The sensor can acquire 
stereo images in across-track direction at 2.5m ground 
resolution in panchromatic mode. The time difference between 
two successive acquisitions in stereo mode of the same area 
depends on the incidence angle, with a minimum of one day 
difference. The satellite flies at a mean height of 832 km, along 
a quasi-polar and sun-synchronous orbit. 
Due to the scientific, social and cultural interest of the project, a 
B/W stereo pair over Bamiyan was provided at special 
conditions by the ISIS Program [ISIS]. The two scenes have 
been acquired on 18" December and 19^ December 2003, at 
10:40 a.m. and 10:20 a.m. local time respectively. The scenes 
are 24000 x 24000 pixels large and cover a mountainous area of 
approximately 60x60 km, centered at (34°50° N, 68? 7 E) and 
(34°50¢ N, 67? 48' E). The cloud cover was zero and the ground 
resolution is 2.5 m. 
Furthermore, a B/W Geo level IKONOS image mosaic over the 
Bamiyan area was provided by Space Imaging [Space Imaging] 
(Figure 3). The scene was acquired on 15" December 2001 and 
covers an area of 11.6x18.5 km, centered at (34°46 N, 67°49’ 
E). The image size is 13957 x 21118 pixels and the ground 
resolution is 1m. 
     
    
Figure 3: The two empty Buddha niches as observed by 
IKONOS (courtesy of Space Imaging, Inc.) Left: Great 
Buddha. Right: Small Buddha. 
2.2 GPS measurements 
For the georeferencing of the satellite images, seven GPS points 
were measured (Figure 4) during our field campaign in August 
2003. Two Trimble GEO Explorer receivers logging carrier 
phase data (C/A code) were used; one receiver was set as 
‘master’ (fix position) and the other served as ‘rover’. The 
points were identified on the IKONOS image and then 
measured in C/A mode. 
After the data collection, the observations were post-processed 
to generate more accurate positions. It was not possible to 
perform absolute differential corrections as the closest. master 
station was ca 1500 km away. Table | summarises the GPS 
measurements. 
Table 1: GPS measurements and related parameters. 
  
  
  
No.of | Average Average Horiz. Vert. 
Points | satellites PDOP Accuracy Accuracy 
7 6 2.78 0.11 m 0.3 m 
  
  
  
  
  
  
  
Figure 4: The distribution of the GPS ground control points on 
the IKONOS image mosaic. In the centre the master station 
position. 
2.3 Scenes orientation and DTM generation 
The DTM was generated from level 1A scenes (radiometrically 
but not geometrically corrected) using SAT-PP (SATellite 
Precision Positioning) software developed at our Institute. 
SAT-PP includes different modules for the precise processing 
of high-resolution satellite image data and allows image pre- 
processing, orientation, matching, DTM/DSM generation and 
object extraction. For more details, see [Zhang, Gruen, 2004; 
Poli et al., 2004]. The IKONOS mosaic orientation was based 
on a 2D affine transformation. On the other hand, the SPOT 
scenes orientation was based on a rational function model. 
Using the camera model, the calibration data and the ephemeris 
contained in the metadata file, the software estimates the RPC 
(Rational Polynomial Coefficients) for each image and applies a 
block adjustment in order to remove systematic errors in the 
sensor external and internal orientation. The scenes’ orientation 
was performed with the help of the GCPs measured with GPS. 
Table 2 shows the image orientation results. 
Table 2: Image orientation results. 
  
  
  
  
  
Source RMSE RMSE | RMSE 
East (m) North (m) Height (m) 
IKONOS 0.56 0.48 - 
SPOT Image Pair 1.22 2.01 1.50 
  
  
  
  
  
    
  
   
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