The HRSC Level 3 Mosaic of Mars - now with mid-latitudes

Introduction:

The HRSC camera onboard the ESA‘s Mars Express spacecraft has been operational in Mars Orbit since January 2004. Since then, it has been acquiring image data of the Martian surface in colour and photogrammetric stereo, achieving a near-complete coverage at resolutions better than 50 m, and complete coverage at lower resolutions.

One of the main mission goals is the creation of a global high-resolution image mosaic, using the HRSC global Digital Elevation Model (DEM) as a basis for geometric correction [1]. In addition, we are creating a mosaic based on the Level-3 images, which are orthorectified using the MOLA DEM. These images are created automatically after data acquisition. While they possess a significantly lower geometric accuracy than the HRSC DEM based level 5 images, they are available much earlier.

The idea of the Level 3 image mosaics is to use this independent processing chain to produce a high-quality image product - with lower resolution - at a much faster pace than it would be possible when relying on the labour-intensive Level 5 data. Using Level 3 data, a complete HRSC mosaic of Mars Chart MC quadrangle can be finished within one month. In the last months, we finished this Level-3 mosaic for the Martian mid-latitudes and equatorial regions, and created a consistent, high quality dataset.

Image 1: The HRSC Level-3 Mars mosaic (MC-30 scheme). New mid-latitude mosaics are outlined in red.

Methods:

Image selection: Images are selected based on the Mars Charts MC-30 scheme.

Lambert correction: A Lambert correction is applied to each image to correct different lighting conditions, planetary curvature etc.

Brightness adjustment: For brightness adjustment, an external brightness reference is used [2,3]. This ensures a consistent brightness for all images of a location regardless of imaging conditions.

Sorting and Adjustment: Initially, all images are sorted by projected resolution. As this does not necessarily reflect image quality, a parameter „effective ground resolution“ was introduced. Using QGIS, images are sorted according to their perceived quality, if necessary, a new effective resolution is assigned manually. A contrast adjustment factor is also assigned, to ensure a good usability of the mosaic for optical interpretation. Along-track, different contrast adjustment values can be assigned. All these changes are recorded in the QGIS project file, which is used by the processing software to provide both image sequencing and contrast adjustments for the next iteration.

Master catalog: All changes to the “effective ground resolution” and contrast adjustments for all images are recorded in a master catalog document. This master catalog allow to process a HRSC mosaic for any region on Mars, providing the settings required for an optimal sequence and contrast. If a new mosaic is created where an adjacent mosaic already exists, adjustments made for images which are part of both regions are applied. These changes can then be revised and adjusted.

Finalizing: The process of sorting and adjustment takes several iterations. For the final version, a mosaic with a fixed resolution of 50 m/pixel is produced. The image borders are processed with a feathering algorithm to minimize visual differences. The final image mosaics are then uploaded to the HRSC mapserver at Freie Universität Berlin [4] and made available as a COG file (cloud-optimized GeoTIFFs [5]) for direct integration in a GIS without download.

Image 2: Mawrth Vallis (MC-11). Image width is approximatey 600 km.

Results:

The resulting Level 3 mosaic provides a unique high-resolution view of the Martian surface between 60° north and 60° south. The resolution surpasses that of the available global THEMIS IR mosaic. It has been referenced to an albedo map, which offers two distinct advantages: Brightness is linked to the TES reference and thus reflects the real visual brightness of the Martian surface, and the different images are homogeneous and offer a possibility for image analysis without accounting for brightness breaks.

The Master catalog for Level 3 data can also be used when creating mosaics based on higher processing levels. These mosaics just require some fine-tuning to compensate for different coverage. For the production of Level-5 mosaics, it has turned out that only minor adjustments are required to create a high-quality image.

The Level 3 mosaics are geometrically inferior to higher-level products, leading to some offsets along image borders. The overall image quality is very good, and the lower geodetic requirements for the image data lead to a more complete coverage.

Outlook:

We are currently using the Master catalog data to create Level 5 mosaics. In addition, we investigate how to transfer the method to polar mosaics with their specific difficulties, and how the methods and tools can be applied to mosaic creation using other datasets.

 

Acknowledgments: The authors wish to thank the DLR for funding this work by DLR grant 50OO2204 (Koregistrierung), and ESA for the continued support and operations of the Mars Express mission

 

Data availability: The data and the current coverage is available on the HRSC Mapserver at Freie Universität Berlin via the following links:

COG file:

https://maps.planet.fu-berlin.de/level3/hrsc3-mos.tif

ArcGIS VRT file:

https://maps.planet.fu-berlin.de/level3/hrsc3-mos.vrt

 

References:

[1] Gwinner et al. (2016), P&SS 126, DOI: 10.1016/j.pss.2016.02.014

[2] Michael, G.G. et al. (2016), P&SS 121, DOI: 10.1016/j.pss.2015.12.002

[3] Michael, G.G. et al., 2025 Icarus 425, DOI: 10.1016/j.icarus.2024.116350

[4] Walter, S.H.G- et al., 2018 ESS 5, DOI: 10.1029/2018EA000389