[X] My Assistant

[Bjorn Jonsson]

It works!! For months I've been trying to get my program for generating DEMs from stereo images to work properly. A recent 'rant' of mine can be found here.

Well, to make a long story short my software now at last works:



The only drawback is that the DEM has a somewhat 'contoured' appearance. This shows up in renderings of the DEM, even after I smoothed it considerably using Photoshop. Still this is much better than the infamous horizontal lines I got in my shape from shading experiments. A rendering showing the DEM:



However, 2-3 meters away from the monitor this looks very nice (the thumbnail image also looks nice ). It turns out that the software has managed to detect most of the craters except for the really small ones. If someone finds a way to process the DEM image and remove the 'contours' without excessive loss of details I'd like to know. Please note that the DEM is a 16 bits/pixel image.

Some additional notes on the DEM: The DEM is a section from a 4320x2160 pixel simple cylindrical map and has an altitude range of approximately 16 km. The lowest points (black) are 778.6 km from Rhea's center and the highest ones (white) are 794.5 km from Rhea's center. This is odd (Rhea's dimensions are 768x763x763 km) and is probably due to inaccuracies in the viewing geometry information that came with the PDS files I used. The overall lower elevation of the upper part of the DEM compared to the lower part is probably another manifestation of this.

I used these two images obtained in January 2005:





North is to the right in the images.

Looking at the images it is apparent that the DEM's heavily contoured appearance at upper left is (probably) due to the fact that there wasn't a very big difference in the viewing geometry in that area whereas in the lower part of the DEM the difference is significant.

Taking a section of Steve Albers' map and draping it over the DEM produces interesting results (The crater at ~(315,260) in the DEM is at ~(3440,1160) in Steve's map). Here is a quick-and-dirty animation:

Rhea experimental DEM animation (4.6 MB)

In contrast, without Steve's map things get less realistic:




However, now the lightsource can be anywhere.

Needless to say I'm happy. The only problem is the huge number of DEMs I want to do: All of Saturn's major satellites (everything that can be done - global if at all possible), various sites on Mars etc. This is a lot of work, the DEM of Rhea required 100+ manually measured control points and then took almost 4 hours to compute on a 3 GHz machine. Also the viewing geometry needs to be refined.

[Bjorn Jonsson]

Here is a fairly big DEM of Mimas:



This is a section of a 1800x900 pixel simple cylindrical map. It covers latitudes -76.6 to 47.6 and longitudes 158.4 to 283.2. The distance from Mimas' center ranges from 186.8 (black) to 209.6 km (white).

This DEM is a mosaic of two DEMs derived from one stereo pair each. One problem is immediately apparent: A ~2.5 km altitude difference near the left edge of the seam. This is due to viewing geometry errors. I am using significantly more accurate viewing geometry parameters than in the PDS files but they are still not accurate enough. At the moment I don't know how I'm going to correct this.

Comparing this DEM to the DEM of Rhea is interesting, for example comparing the crater depth/diameter ratios.