Frequently asked questions

HRSCview is a web interface to the Mars Express HRSC image archive at the Freie Universitaet Berlin. It carries out on-the-fly image subsetting, stretching, compositing, and the the case of perspective views, projection of map-projected data-products. Access to the images is provided through a footprint map, by coordinates or by image identifier. The site is provided jointly by the Freie Universitaet and the German Aerospace Center (DLR).

Which images are available through HRSCview?

HRSCview provides access to the released images and derived data products of the Mars Express HRSC experiment. New data are released at six-monthly intervals.

What is the difference between the preliminary and archival stereo image processing?

Preliminary 200m DTMs and preliminary orthoimages are generated by a fully automatic processing sequence. The first priority for these data products is fast availability, being produced soon after data acquisition for the initial assessment of new data. No quality control is applied, orientation data is not improved by bundle adjustment and is not necessarily based on the final version provided by ESA. The procedure is described in more detail here:

F. Scholten, K. Gwinner, T. Roatsch, K.-D. Matz, M. Wählisch, B. Giese, J. Oberst, R. Jaumann, G. Neukum, and the HRSC Co-Investigator Team: Mars Express HRSC Data Processing - Methods and Operational Aspects, Photogrammetric Engineering & Remote Sensing, Vol.71, No. 10, pp.1143-115, 2005.

Archival HRSC DTMs (up to 50m resolution) and orthoimages are generated at full resolution and based on the archival set of input data and on improved orientation data. The derivation of archival data products is being carried out systematically for all HRSC observations and is currently ongoing. More information on the specifications and the generation of archival HRSC DTMs and orthoimages can be found in the following:

K. Gwinner, T. Roatsch, K.-D. Matz, F. Scholten, S. Elgner, F. Preusker, J. Oberst, R. Jaumann, D. Heather, and G. Neukum. Archival stereo data products of the HRSC experiment onboard Mars Express LPSC XXXIX, #2373, 2008.

K. Gwinner, F. Scholten, R. Jaumann, T. Roatsch, J. Oberst, G. Neukum: Global mapping of Mars by systematic derivation of Mars Express HRSC high-resolution digital elevation models and orthoimages ISPRS IV/7 Extraterrestrial Mapping Wokshop, Houston, 2007.

Which reference system is used for the elevation values?

For those observations where an archival DTM has been produced, the elevations are shown relative to the MOLA equipotential surface (areoid), so should be directly comparable with MOLA topography values. For the orbits which have only preliminary DTMs, the elevations are scaled relative to the minimum value within the displayed scene.

How is the slope calculated?

The slope is calculated over a two-displayed-pixel baseline (i.e. dz=(z[x+1]-z[x-1])/(2*dx) ) where the displayed pixels are larger or equal to the DTM pixels; where the displayed pixels are smaller than the DTM pixels, the slope is calculated over a two DTM-pixel baseline.

What does Mars-like colour mean?

The Mars-like colour option applies a fixed stretch to the raw colour image which enhances the red and diminishes the blue channels. The result is an approximation to the expected Mars colour as seen from the surface, but shouldn't be taken as definitive.

Sometimes there are vertically striped regions in the images or abrupt changes in colour. Why?

For various reasons, gaps occasionally occur within the data acquired by HRSC. If they occur in the nadir channel, they are typically filled with interpolated pixels which may appear as stripes. If they occur in the colour channels, this can lead to hard colour boundaries.

In some places there seem to be relief features which don't correspond to the image. Why is that?

The elevation model is derived from stereo matching, a technique which works very well in regions with many contrasting edges. In smooth, featureless areas, or when the surface contrast is reduced by atmospheric dust, it becomes harder to match the stereo channels (the human eye has the same difficulty), so these areas have to be interpolated. Spurious matches in low-texture areas can cause artefacts in the generated elevation model.

Where can I find the full data products?

The Mars Express HRSC images are available for download from the ESA Planetary Science Archive. The archive contains images which have been radiometrically corrected (level 2) and map-projected onto a MOLA DTM (level 3). Many images which have been ortho-rectified using a high-resolution HRSC stereo-derived DTM (level 4) are now available from the Freie Universitaet website. Direct links to the data in the PSA are provided from the HRSCview product pages. It should be noted that, although the HRSC products in the ESA PSA are in PDS format, at present those available through the FUB site are in VICAR format (compressed with bzip2).