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AUTOMATED DIGITAL TOPOGRAPHIC MAPPING TECHNIQUES FOR MARS 
T. Day, A.C. Cook and J-P. Muller 
Department of Photogrammetry and Surveying, 
University College London, Gower Street 
London. WC1E 6BT, UK 
e-mail: tday@ps.ucl.ac.uk, tcook@ps.ucl.ac.uk or jpmuller@ps.ucl.ac.uk 
Abstract 
An automated system for the production of digital el- 
evation models from Viking Orbiter imagery has been 
developed by combining the Otto-Chau adaptive sheet- 
growing least-squares stereo-matcher and the Frankot- 
Chellappa shape-from-shading algorithm. Accuracy as- 
sessment results derived by comparison with manually 
derived photogrammetric contours from USGS Flagstaff 
are presented. The image selection criteria for process- 
ing large numbers of VO images is described. 
Keywords: Extraterrestrial, Image Matching, Auto- 
mated Mapping, Photogrammetry, Stereoscopic 
1 Introduction 
The EXODUS (EXtra-terrestrial Orbital DEMs for Under- 
standing Surfaces) Project was initiated with the intention 
of applying automatic stereo-matching techniques devel- 
oped for SPOT imagery to orbital images obtained by the 
Viking Orbiter probes for the purpose of constructing high 
resolution DEMs of Mars, including a global Hs (m 0.5km) 
DEM [Muller et al. 92]. This project builds on previous 
work to develop an automated Digital Elevation Model 
(DEM) extraction system for spacebourne and industrial 
stereo imagery. 
2 Stereo-matching 
The Otto-Chau stereo-matcher [Otto and Chau, 89], and 
its successful application to SPOT imagery, has been doc- 
umented previously in [Day and Muller, 88], [Muller, 89]. 
The matcher requires at least one initial seedpoint to com- 
mence sheet-growing. At the present time seed-points are 
generally chosen manually for each image pair to be pro- 
cessed, although initial experiments have been carried out 
with automated seedpoints based on the Foerstner oper- 
ator [Allison et al. 91]. 
The application of the stereo-matcher to the eastern 
half of the South-East Aeolis mapsheet (MC-23 SE) and 
the Tithonium Chasma area of Valles Marineris is demon- 
strated. 
2.1 South-East Aeolis 
2.1.14 Image Selection Criteria 
Viking Orbiter only had serendipitous stereo coverage as 
no dedicated stereo-coverage was made. In addition, al- 
801 
though most of the planet is covered with images of IFOV 
x 250m, only a small percentage of this area is covered 
with stereo at this resolution. The Mars Information Sys- 
tem (MIS) [Cook et al. 92] was used to select the image 
pairs suitable for mapping of this area. The MIS holds data 
on the EXODUS Project's archive of 13472 Viking Orbiter 
images at various levels of radiometic [Batson, 87] and 
geometric [Edwards, 87] processing obtained from USGS 
Flagstaff, Goddard Space Flight Center, JPL and the Uni- 
versity of Hawaii. We limited the number of possible pairs 
by applying the following chosen constraints: 
Overlap The area of stereo coverage within the area 
of interest must be at least ten percent of the total 
ground area covered by the two images. 
Viewing angle The spacecraft must be at least 20? 
above the horizon at the centre of the image. 
Illumination difference The difference in solar altitude 
must not exceed 10? and the difference in solar az- 
imuth must not exceed 45? (azimuth difference from 
[Blasius et al. 80]. We do not simply place an up- 
per limit on the angle between illumination vectors as 
this could admit pairs where e.g a ridge is illuminated 
from opposite sides. 
Resolution Ratio The ratio of image resolutions must not 
exceed 2.5 : 1, which appears to be the limit at which 
stereo matching is possible with Viking imagery and 
the Otto-Chau matcher. 
Precision (computed as pixel IFOV divided by B/H ratio) 
must be better than 1km. 
Camera model quality An image may have control 
points or good camera pointing information. There 
must be sufficient image navigation parameters to 
ensure accurate absolute orientation. 
Limb Neither image must contain the limb of Mars. 
Radiometric Correction At least one image of the pair 
must be radiometrically corrected to allow the possi- 
ble application of shape-from-shading techniques. 
Filter Both images must have been taken with the same 
colour filter in place. 
Image quality A record of unuseable images is kept; this 
eliminates images discovered to contain e.g dust- 
storms, clouds and motion blur. 
A search for image pairs satisfying these criteria re- 
turned 17 image pairs for the South-East Aeolis region. 
Selecting only image pairs in which at least one image 
has a resolution of 650m or better resulted in only 4 pairs.