Full text: XVIIIth Congress (Part B4)

  
(f) Terrain-corrected_ortho_images derive from the raw 
images, resampled to the proper terrain geometry. One 
will obtain geocoded ortho-rectified radar images from 
each of the 3 cycles. An individual surface point may 
thus be shown on different images. Due to Magellan’s 
quasi-polar orbit, near to the pole a location may have 
been imaged tens to hundreds of times. 
(g) Shape-from-shading models how a human observer 
would interpret brightness and shadows in the radar 
image, which roughly appears like an oblique 
illumination of the terrain. Unjustified, this process is 
assuming uniform backscatter-properties for the whole 
surface of the planet and accuracies are much 
dependent on the look-direction of the sensor. Still, a 
shape-from-shading approach applied to geocoded 
images is beneficial to refine details of the digital 
terrain elevation beyond what can be extracted from 
the geometric disparities in stereo pairs. 
(h) An image and data stack may consist of an optimum 
digital elevation model and a set of overlapping 
geocoded and ortho-rectified images from multiple 
cycles. One may organize the material into an image 
data base as discussed in Section 3. 
5 COPING WITH DISSIMILARITY 
5.1 Opposite Side Images 
Subsection 4.3 assumed that all the 3 Cycles of Magellan 
images can be geocoded using terrain elevations extracted 
from stereo or from altimetry. This is only true if the 
images from the third non-stereo (,opposite-side‘“) Cycle 
can successfully be processed with the elevation data. 
5.2 Experiments with Image Simulation 
One software tool for this task is simulation. The opposite- 
side image can be synthesized using the topographic relief 
available from altimetry, stereoscopy or from shape-from- 
shading. Figure 4 illustrates the problem of an opposite- 
side Magellan pair and relates this to a DEM obtained by 
stereo. Simulating the opposite-side image produces an 
intermediate product that may be matched with the actual 
opposite-side image by conventional methods. So, one can 
verify if and where the two images are geometrically 
identical. If not, then the digital elevation model needs to 
be corrected so that identity between the simulated and 
real image is accomplished. This problem domain is the 
subject of ongoing work (Kellerer-Pirklbauer, in print). 
  
Figure 4: Opposite-side Magellan images of accentuated 
terrain. Look-angles are 43? East and 25? West. 
Location at 2°S. Area covered is 30 km x 30 km. 
495 
6 CONCLUSIONS AND OUTLOOK 
We have described geometric processing of Magellan 
radar images. The Internet provides access to the images 
in the Planetary Data System PDS. A systematic effort 
currently converts the full resolution original image strips 
into a final image product, the F-MAP. However, those 
images still are affected by the errors of the preliminary 
ephemerides obtained during the Mission, and they just 
corrected using the poor DEM obtained from altimetry. 
We have proposed and are actively pursuing the 
development of a suite of modules that would represent an 
alternative system for reprocessing the raw phase histories. 
We work towards a final image data base of ortho-rectified 
and geocoded images. The DEM would be produced from 
stereo, taking into account also the opposite-side imagery. 
We are addressing a number of individual problems such 
as the database issues of managing the large 400 Gbyte 
images from Venus; the difficulty one faces because of the 
excessive layover in topographically accentuated terrain 
which occurs because of the very steep look-angles of 
Magellan; the matching of opposite-side image using the 
topographic elevations obtained from stereo; the use of 
shape-from-shading and of image simulation that should 
alleviate the concerns resulting from layover and 
dissimilarities in opposite-side images. We are also 
concerned about computing times and therefore have 
embarked on an effort to parallelize key algorithms such 
as shape-from-shading, image matching, surface point 
gridding, and perspective rendering for quality control 
(Goller et al., in print; Goller, in print). 
The Magellan mission was terminated in 1994; the 
imaging portion of the mission ended already two years 
earlier in 1992, after the imaging radar had operated for 3 
years. Since the end of the Mission the data analysis has 
become a coordinated but modest effort, much reduced 
from that which existed during the Mission. It is through 
the Planetary Data System and its participating 12 
institutions that the data are being processed at the rate 
which funding supports. We hope that radargrammetric 
processing will evolve into a complete suite of software 
elements to produce an accurate DEM and associated 
ortho-rectified images. The basic prerequisite is an 
improved ephemeris which is outside the scope of the 
radargrammetric work. It will be the pivotal element to 
make the geometric reprocessing effort worthwhile. 
ACKNOWLEDGEMENT 
This work has been supported in part by the Austrian 
Research Foundation FWF (project S 7001), and by a 
grant from the Austrian Academy of Sciences. 
REFERENCES 
Connor C. (1994) Determing Heights and Shapes of Fault 
Scarps and Other Surfaces on Venus Using Magellan 
Stereo Radar. Manuscript, Princeton Univ. Dept. of 
Geological Sciences. 
Ford Pl, G. Pettengill (1992) Venus Topography and 
Kilometer-Scale Shapes. J. Geographical Research, Vol. 
97, pp.13103-13318. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996 
 
	        
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