tanbul 2004
COMPARISON OF THE ACCURACY OF DEM FROM SPOT HRS TWO-FOLD STEREO DATA
AND HRS/HRG THREE-FOLD STEREO DATA IN BARCELONA TEST SITE
Rupert Müller, Peter Reinartz, Manfred Lehner, Manfred Schroeder
German Aerospace Center (DLR), Remote Sensing Technology Institute
PO Box 1116, D-82230 Wessling, Germany
rupert.mueller@dlr.de
WG 1/2
KEY WORDS: Stereoscopic data, image matching, two-fold stereo, three-fold stereo, DEM generation, accuracy analysis
ABSTRACT:
ISPRS and CNES announced the HRS (High Resolution Stereo) Scientific Assessment Program during the ISPRS Commission I
Symposium in Denver in November 2002. 9 test areas throughout the world have been selected for this program. For the Barcelona
test site not only two-fold stereo HRS data but also HRG data of the same region have been acquired. This leads to the possibility to
compare the DEM production and accuracy differences between two-fold and three-fold stereo data sets. For the derivation of the
DEMs, the DLR own stereo processing software, developed for the MOMS-2P three line stereo camera is used. The matching
process using three images is more robust than with two images due to the possibility to eliminate blunders by matching three stereo
pairs. Furthermore the forward intersection of the three image rays allows better constraints for the object space intersection. Due to
these reasons the DEM filtering in object space that is necessary for DEMs from two-fold stereo is very much reduced in the case of
three-fold stereo data. The derived DEMs are compared with the high precision reference DEM by several methods regarding:
height accuracy, location accuracy, blunders, error budget depending on surface properties etc...
1. INTRODUCTION
Within the HRS Scientific Assessment Program, which gives a
user community the possibility to derive digital elevation
models (DEM) from along track stereo data from space with the
new HRS sensor on SPOT 5 (Gleyzes 2003), the performance
of different methods should be tested. This paper deals with the
comparison of DEM, derived from HRS two-fold stereo data
and HRS/HRG three-fold stereo data. Main emphasis is placed
on the use of the very accurately determined absolute and
relative position and attitude data delivered along with the
image data.
HRS produces image stereo pairs with two optics looking
forward and backward (£20 degrees) with respect to the nadir
direction. The camera has a spatial resolution of 10 meter across
track and along track, but a ground sampling distance of about 5
m along track for obtaining higher resolution of the parallaxes
for the DEM generation. The swath of the HRS is 120 km
(12000 CCD elements) and one acquisition sequence extends
600 km along track. HRG produces two staggered panchromatic
nadir looking images (HMA and HMB) with a resolution of 5 m
each, which can be combined to a 2.5 m resolution image.
2. TEST AREAS AND GROUND REFERENCE DATA
The test area is located in Catalonia (Spain) and includes the
city of Barcelona, covering dense urban areas as well as
mountainous terrain. The data of Catalonia have been acquired
on October 15" 2002 with a sun elevation of 39? and no clouds.
The data provided by SPOT IMAGE contain the following
parts:
e Two sets of 8 bit panchromatic image data (size
12000 x 12000 pixel = 120 km across x 60 km along
track) of Catalonian test area from two viewing
directions in TIF format
e Two sets of 8 bit HGR panchromatic nadir looking
images (HMA and HMB data) with a ground
resolution of 5 m x 5 m of part of the test site (size
12000 x 12000 pixel = 60 km across x 60 km along
track )
e XML-files containing all additional information
regarding time synchronization, position (DORIS),
attitude (ULS: star tracking unit to compute absolute
orientation in a celestial frame combined with the
attitude orbit control system AOCS), interior
orientation (tabulated look angles for each pixel)
e ASCII text files containing information on the
delivered data.
The ground reference data are the following:
e 32 color orthoimages (1:5000) with pixel size of 0.5
meter and accuracy better than 1 pixel (16)
e DEM with pixel spacing 15.0 meter and orthometric
height accuracy of 1.1 meter (10)
Both reference data sets (orthoimages and DEM) are provided
in UTM zone 31 with the geodetic datum ED50 (European
Datum 1950).
3. PREPROCESSING OF THE ANCILLARY DATA
The delivered SPOT 5 HRS Level 1A product consists of the
image data in standard TIF format and the meta data in DIMAP
format. The following information is extracted from the XML
ancillary file for further processing:
e the ephemeris data containing position and velocity of
the satellite measured by the DORIS system every 30
