Digital and Automated High Resolution Stereo Mapping of the Sonnblick Glacier (Austria) with HRSC-A

The airborne High Resolution Stereo Camera (HRSC-A) is a multiple line scanner which simultaneously acquires stereo and colour information with absolute spatial accuracy in the decimetre range. The multiple stereo principle is particularly well suited for obtaining precise topographic and imaging data of rugged terrain as found in mountainous regions: a permanently nadir-looking stereo channel provides favourable visibility conditions even for steep slopes and minimises the occurrence of shadow zones which affect the performance of side-looking sensor systems such as SAR. A sophisticated and fully automated photogrammetric processing system allows to determine topography even in areas which are problematic for image correlation techniques needed to extract height information from stereo imagery, e.g. in areas with low texture information like snow. In order to supplement existing monitoring techniques for snow and ice-covered regions, HRSC-A was used in a pilot study to map an area of the Hohe Tauern mountain range in Austria enclosing some of the most intensively analysed glaciers in the Alps. The results are of outstanding quality and suggest the future application of HRSC-A for the remote sensing of snow and ice, particularly in the context of climate-related studies. INTRODUCTION The Stubacher Sonnblickkees (Hohe Tauern, Austria) is one of the most intensively monitored glaciers in the Eastern Alps (Slupetzky and Aschenbrenner 1998). Its length has been measured for 40 years, and its mass balance has been determined since 1963. Moreover, the water budget in the area is monitored within the drainage area of an artificial reservoir. Mass balance monitoring is focused on the understanding of climate-glacier interactions. It is part of the International Hydrologic Program (IHP) of UNESCO and represents a contribution to the Global Environment Monitoring System (GEMS) of UNEP. Since glaciers are sensitive climate indicators, continuous long-term monitoring is increasingly important for the interpretation of global changes. Regular mapping is essential to provide quantitative results and to allow for trend extrapolation. Here, we address the challenge of fully automated and digital airborne mapping of glaciers in an alpine environment. METHODS Instrument: The High Resolution Stereo Camera (HRSC) has originally been designed for space applications and will be flown onboard the ESA Mars Express mission in 2003 (Neukum and Tarnopolsky 1990; Albertz et al. 1992; Neukum et al. 1999). An airborne version of this camera (HRSC-A) has been applied successfully in many flight campaigns (Neukum 1999), including 3Dimage data provision for volcanology, agriculture, forestry, open coal mining and mapping of urban areas. Pilot experiments on active volcanoes have previously demonstrated the potential of the camera system for mapping rugged terrain (Gwinner et al. 1999). The sensor system is based on the along-track multiple-stereo pushbroom principle (see Figure 1), meets all requirements of a photogrammetric sensor, and combines 3D-capabilities and high spatial Proceedings of EARSeL-SIG-Workshop Land Ice and Snow, Dresden/FRG, June 16 – 17, 2000 EARSeL eProceedings No. 1 247 resolution with multi-spectral data acquisition (Table 1). Nine CCD lines in the focal plane behind one single optics are precisely calibrated to allow for high geometric accuracy. Four lines are equipped with spectral filters (IR, Gr, Bl, Rd in Fig. 1). The unique multi-stereo functionality is based on the other five lines (SA, PA, Nd, PF, SF), providing panchromatic data with specific viewing angles. Variable resolutions can be generated, resulting in ground pixel sizes as small as 10 cm from an altitude of 2500 m. During flight operation, the camera is mounted on a stabilised platform in order to damp mechanical vibrations and to enforce near-nadir viewing geometry. An integrated GPS/INS navigation system (APPLANIX POS/DG, see Hutton and Lithopoulos 1998) including a GPS receiver and a strap-down INS is used to obtain high accuracy measurements of the exterior sensor orientation. Figure 1. Imaging Principle of HRSC-A Photogrammetric Processing: A digital photogrammetric processing system was developed for the HRSC experiment on the Mars96 mission in co-operation with the Technical University of Berlin. An automated procedural software system has been derived for the airborne operation of HRSC-A (Figure 2). The photogrammetric processing line makes use of a set of systematically preprocessed image, orientation and calibration data. Since the Inertial Measurement Unit (IMU) cannot be mounted exactly parallel to the HRSC-A camera axes, the offset between both systems has to be computed. This computation can be done without any additional ground control information, since HRSC-A provides sufficient multi-stereo image information. Table 1. Technical Parameters of HRSC-A Image data of a pushbroom system like HRSC-A are influenced by the continuously changing exterior orientation. Applying image matching techniques for the generation of Digital Elevation ModHRSC-A/QM Technical Parameters Focal Length: 175 mm Total Field of View: 37.8° x 11.8° Number of CCD Lines: 9 Stereo Angles: ±18.9° and ±12.8° Pixels per CCD Line: 5184 (active) Pixel Size: 7 μm Radiometric Resolution: 10 bit reduced to 8 bit Read-Out Frequency: 450 lines/s Mass: 12 kg (32 kg including subsystems) Proceedings of EARSeL-SIG-Workshop Land Ice and Snow, Dresden/FRG, June 16 – 17, 2000 EARSeL eProceedings No. 1 248 els (DEM) to the original image data would result in failures, since the matching algorithm would match not only textures but also flight motion effects, especially when they appear periodically. To avoid these failures, the HRSC-A data of all five stereo sensors are pre-corrected by rectifying them to a mean terrain level based on the data of the exterior orientation. Figure 3 shows the potential of the rectification process and the quality of the GPS/INS data, even under extreme conditions during a flight manoeuvre. The permanent nadir viewing geometry of HRSC-A makes the nadir channel the most appropriate master image. It guarantees the best possible coverage (with respect to completeness) because of the reduction of hidden areas. In order to reduce the influence of matching failures and to introduce redundancies, the nadir grid points should not be matched with only one partner but with all (four) possible stereo partners. Thus, the multi-stereo capability of HRSC-A provides the possibility to determine points with up to five observations and to eliminate errors. Installation Offset between HRSC-A and IMU Simultaneous Determination of Position and Attitude Orientation Data for each Image Line Geometric Correction of Image Data Image Matching Orthoimage Generation DEM Follow-Up Products Colour Orthomosaics DEM Generation Orthoimage Mosaics HRSC-A Image Data, Calibration Data, D-GPS & INS Data Figure 2. HRSC-A Photogrammetric Processing Line Figure 3. Effectiveness of geometric correction under extreme conditions. Original HRSC-A data (lower left), pre-rectified image data (centre) and enlargement of pre-rectified image (upper right). Flight altitude was 3000 m, ground resolution 15 cm (note that the white lines of a tennis court are quite visible in the enlargement). Together with information about the interior and exterior orientation each set of image co-ordinates of a match point includes up to five rays. The intersection of these rays and the quality of intersection is computed within a least-squares adjustment process. Points defined by less than three rays Proceedings of EARSeL-SIG-Workshop Land Ice and Snow, Dresden/FRG, June 16 – 17, 2000 EARSeL eProceedings No. 1 249 are eliminated due to the lack of redundancy. The remaining set of object points is defined in the GPS/INS reference system (WGS84) and can be transformed to other geodetic datum, depending on the intended cartographic representation of the final raster DEM. The DEM, represented in any requested map projection, can now be interpolated from the set of object points. It can be used for the extraction of profiles, contour lines or other DEM follow-up products and is the basic prerequisite for the subsequent generation of orthoimages. During orthoimage generation, rays defined by the calibration and the orientation data of each pixel are intersected with the surface described by the DEM. The final step within the photogrammetric data processing is the generation of orthoimage mosaics using orthoimages of adjacent strips. The result is a homogeneous image mosaic for each spectral band, represented in the requested map projection. Potential of the HRSC-A for Cartographic Applications: Based on the digital concept of the camera and its associated digital photogrammetric processing line, the HRSC-A system enables various cartographic applications. On the one hand, different map scales can be derived with appropriate accuracy. However, not only high-resolution and highly-accurate data are the main advantages of the system, the multi-spectral image data can also be used directly for various thematic representations, especially combined with the 3D-capabilities of the system. Table 2 gives an overview of the ground resolution and accuracy of HRSC-A image and 3D-data and the resulting capabilities for different map scales. If high-resolution panchromatic data are merged with multi-spectral data (e.g. by using HSI-color transformation), the resulting data set includes both high resolution and multispectral information and can be used for nearly the same map scales as given for panchromatic data in Table 2. Table 2. Resolution, Accuracy, and Cartographic Capabilities of HRSC-A Image Data The completely digital and highly automated imaging and processing line enables the unique derivation of high-resolution and multi-spectral image and 3D-data, combined with short processing periods. Even project areas of several hundred