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/13150.pdf 
SPOT-5/HRS STEREO IMAGES ORIENTATION 
AND AUTOMATED DSM GENERATION 
Daniela Poli, Zhang Li, Armin Gruen 
Institute of Geodesy and Photogrammetry, ETH Zurich, 8093 Zurich, Switzerland 
TS HRS(2) - DEM Generation from SPOT-5 HRS Data 
KEY WORDS: SPOT, Pushbroom, Sensor model, Orientation, Matching, DEM 
ABSTRACT 
HRS (High Resolution Sensor), carried on SPOT-5, is the first high-resolution sensor on the SPOT constellation that enables the 
acquisition of stereo images in pushbroom mode from two different directions along the trajectory. The Institute of Geodesy and 
Photogrammetry (IGP) participated as Co-Investor in the ISPRS-CNES initiative for the investigation on DEM generation from 
SPOT-5/HRS stereoscenes. This paper describes the work carried out at IGP on a stereopair acquired on 1st October 2002 over parts 
of Bavaria and Austria. 
For orienting HRS imagery, two alternative approaches have been used: a rigorous sensor model and a rational function model. Both 
algorithms have been implemented at our Institute. The rigorous sensor model is based on the classical collinearity equations, which 
are extended by the sensor external orientation modeling with 2nd order piecewise polynomials depending on time and by self- 
calibration parameters. Using well distributed Ground Control Points (GCPs), the unknown internal and external parameters are 
estimated with a least squares solution. The alternative method, independent from the camera model, does not describe the physical 
imaging process, but uses rational polynomials to relate image and ground coordinates to each other. This algorithm consists of two 
steps: 1) calculation of Rational Polynomial Coefficients (RPC) for each image with a least-squares using the geometric information 
contained in the metadata files; 2) block adjustment with the computed RPC model using GCPs. Both orientation methods gave 
RMS errors in Check Points (CPs) in the range of 1 pixel in all coordinate directions. 
Using the triangulated orientation elements, the DSM was extracted with algorithms and software packages for CCD linear sensors 
developed at IGP. After the creation of image pyramids, the matches of three kinds of features (feature points, grid points and edges) 
on the original images are found progressively in the next levels starting from the low-density features on the images with the lowest 
resolution. A triangular irregular network based DSM is constructed from the matched points on each level of the pyramid and used 
in turn in the subsequent pyramid level for the approximations and adaptive computation of the matching parameters. A modified 
Multi Photo Geometrically Constrained Matching algorithm is employed in order to achieve sub-pixel accuracy for all the matched 
[catures. The extracted DSM was compared to the reference DSMs obtained from laser data and map contours at different spacings 
(5x5 m, 25x25 m and 50x50 m) provided by DLR in Oberpfaffenhofen using both terrain height and orthogonal distances. The 
results show RMS values between one and two pixels on the average and a systematic error mainly due to the presence of trees. After 
à manual removal of the main areas covered by trees in the reference DSMs sites those errors have been removed. The final results 
show a mean error in the range of 1-5 meters. 
1. INTRODUCTION 
SPOT-5 was launched on 4" May, 2002 by Arianespace from 
the Kourou Space Centre in French Guyana. After completing 
two months of in-orbit tests it became fully operational in July 
2002. 
SPOT-5 belongs to the SPOT (Satellite Pour l'Observation de la 
l'erre) constellation developed by CNES (Centre National 
D'Etudes Spatiales). The constellation consists of 3 operational 
satellites (SPOT-2, SPOT-4 and SPOT-5) flying along a near- 
polar, near-circular and Sun-synchronous orbit at a mean 
altitude of 832 km, an inclination of 98.7 degrees and a mean 
revolution period equal to 101.4 minutes. The SPOT satellites 
orbit the same ground track every 26 days with a nominal cycle 
of 369 revolutions and cross the equator from North to South at 
10:30 a.m. mean local solar time (Gleyzes et al., 2003). 
Within the constellation, SPOT-5 is the most innovative 
satellite. The new HRG (High Resolution Geometry) 
instruments, derived from the HRVIR instrument on SPOT-4 
offer high resolution in across-track direction with up to 2.5m 
resolution in panchromatic mode. Moreover the new HRS 
(High Resolution Sensor) allows the acquisition of stereo 
images in along-track direction, using two telescopes pointing 
about 20 degrees forward and backward (Gleyzes et al., 2003). 
Other payload packages include the same Vegetation instrument 
as on SPOT-4, and the DORIS instrument, for greater orbital 
accuracy. 
The Institute of Geodesy and Photogrammetry (IGP) of ETH 
Zurich joined the HRS Scientific Assessment Program (HRS- 
SAP), organised by CNES and ISPRS. This initiative, 
announced in Denver in 2002 at the ISPRS Commission I 
Symposium, has the aim to investigate the potential of SPOT- 
5/HRS sensor for DEM generation in order to help CNES to 
improve its future Earth Observation systems and all users to 
better know and trust the accuracy and quality of the HRS 
instrument and the derived DEM (Baudoin et al., 2003). 
IGP joined the Initiative as Co-Investigator, that is, it processed 
the data provided by one the Principal Investigators, generated 
two DEMs with two different orientation methods, compared 
them to the reference DEMs and produced a quality report. 
In this paper the work carried out at IGP within HRS-SAP is 
reported. After the description of the available data, the 
processing algorithms applied for images orientation, matching 
and DEM generation are presented. The results obtained after 
the comparison between the generated DEMs with the reference 
ones are reported and analysed. Final comments will conclude 
the paper.