8.9. Mars Express High Resolution Stereo Camera (HRSC)

The HRSC camera on the Mars Express satellite is a complicated system, consisting of multiple channels pointed in different directions plus another super resolution channel. The best option to create DEMs is to use the two dedicated stereo channels. These are pointed ahead of and behind the nadir channel and collect a stereo observation in a single pass of the satellite.

Since each observation contains both stereo channels, one observation is sufficient to create a DEM.

Data can be downloaded from the HRSC node in the Planetary Data System (PDS).

HRSC data is organized into categories. Level 2 is radiometrically corrected, level 3 is corrected and mapprojected onto MOLA, and level 4 is corrected and mapprojected on to a DEM created from the HRSC data. You should use the level 2 data for creating DEMs with ASP. If you would like to download one of the already created DEMs, it may be easiest to use the areoid referenced version (.da4 extension) since that is consistent with MOLA.

8.9.1. Preparing the data

Fetch the two stereo channels using wget from:

https://pds-geosciences.wustl.edu/mex/mex-m-hrsc-3-rdr-v4/mexhrs_4000/data/1995/h1995_0000_s13.img
https://pds-geosciences.wustl.edu/mex/mex-m-hrsc-3-rdr-v4/mexhrs_4000/data/1995/h1995_0000_s23.img
../_images/hrsc_example.png

Fig. 8.7 Sample outputs from a cropped region of HRSC frame 1995. Left: Cropped input. Center: Block matching with subpixel mode 3. Right: MGM algorithm with cost mode 3.

See Section 2.3.1 for how to set up ISIS and download the needed kernels. For HRSC, they are part of the mex dataset.

It appears that hrsc2isis is not able to read the level 3 images that were downloaded above, and PDS no longer offers level 2 images. What seems to work is edit the .img files and change level 3 to level 2. That can be done with:

perl -pi -e 's#(PROCESSING_LEVEL_ID\s+=) 3#$1 2#g' *.img

Then run:

hrsc2isis from=h1995_0000_s13.img to=h1995_0000_s13.cub
hrsc2isis from=h1995_0000_s23.img to=h1995_0000_s23.cub
spiceinit from=h1995_0000_s13.cub ckpredicted=true
spiceinit from=h1995_0000_s23.cub ckpredicted=true

Here we added the ckpredicted=true flag to spiceinit. Adding web=true can help avoid downloading the kernels, if this works. See the (spiceinit documentation).

8.9.2. Running stereo

HRSC images are large and may have compression artifacts so you should experiment running stereo on a small region with stereo_gui (Section 16.70).

The suggested command to run on the full images is:

parallel_stereo h1995_0000_s13.cub h1995_0000_s23.cub \
  --stereo-algorithm asp_mgm                          \
  --cost-mode 3                                       \
  mgm/out

See Section 6 for other stereo algorithms, and information on tradeoffs between them.

A DEM is created with point2dem (Section 16.57):

point2dem                            \
  --stereographic --auto-proj-center \
  mgm/out-PC.tif