1 Background - mountain.PROJECTS

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TURKU UNIVERSITY DEPARTMENT OF GEOGRAPHY PUBLICATIONS B Nr 4A procedure for making close-up digital elevation models of glacier surface was developed inorder to do glaciological change detection. From the multitemporal models it is possible tofollow accurately the changes of different surface features like melt water channels, glaciertables, ablation hollows etc. Results indicate that close-up DEMs can be used to densify moresparse DEMs and to detect changes in ice structure and movements.Figure 25. The terrestrial DEM of Hintereisferner test field in September 2002.Figure 26. The airborne laser DEM from the same day as the terrestrial DEM (see fig. 25).The most complete set of historical glass plate photographs were found from Hochjochfernerglacier located next to the Hintereisferner. This data can be used for validation of thetechnology. The glass plates, 93 in total and each of size of 120 mm x 160 mm, were scannedby the University of Bundeswehr in Munich with a pixel size of 12,5 microns. Thecomparison with validation profiles showed high accuracy of the original topographic mapbeing processed in former times. However, the density of the measured points compared tothe interpolated contour interval is low in upper and remote parts of the glaciers and thisindicates high uncertainty in the topographic records.Processing of aerial photographsSince 1920, aerial photography has been the primary practical alternative to producetopographic models of large glacial areas. As far as it regards past records, the digital DEMshave been produced either indirectly by digitising the topography from old analog maps, ordirectly by digitising the DEMs from old original images. During the project, newphotography was performed in both areas for the purpose of DEM comparison.The processing of images includes principally two parts, the aerial triangulation and theinterpretation of the glacier surface by stereoscopic or multi-image matching. Aerial36
PETRI PELLIKKA AND KARI KAJUUTTI (ED)triangulation based on minimal, or even poor ground control, can be sufficiently supported bycamera based GPS-observations. Automatic surface matching and DEM generation makes theprocessing today highly cost efficient. However, manual and visual checking is stillrecommended.In case of previous photography without any GCP's or airborne GPS data, the blocks can begeoreferenced according to natural GCP's. These can be collected from new photography.However, the accuracy of the natural GCP's is poor. They contain uncertainties both due toblock deformation of the block of the new images and due to the weak network geometry ofnatural GCPs. Within OMEGA, these uncertainties could not be investigated. One possibilityfor this would be to triangulate all blocks simultaneously with the adjustment of newphotography.Acquisition and processing of digital camera dataDigital camera data were used for constructing image mosaics and elevation models. Theelevation models were compared with elevation models constructed with other data types.The image mosaics were used for construction of glacier maps. The data from Engabreenwere acquired using Nikon D1X true colour digital camera and the data from Hintereisfernerwere acquired using ALTM 4K02 false colour camera. The main purpose of the softwareEnsoMOSAIC used for DEM construction is a creation of orthoimage mosaics. The softwareis not the optimal for DEM creation, since the DEM is made only for the orthorectification ofthe mosaic. With numerous new ground control points, the accuracy of the DEM can beincreased, however. It is concluded, that very high resolution image mosaics can be used forstudying the glacier borders and glacier zones, but for topographic mapping the accuracy wasnot satisfactory.The advantage of digital camera data is the flexibility in data acquisition compared to satellitedata. The cost of the data consists of flying hours, operator, equipment and processing and canbe considered as expensive as very high-resolution satellite data. For small and close glaciersdigital camera data is evidently a cost-effective data type to use. Because the sensor isairborne sensor, the geometric resolution can be varied from 20 cm to several metres. Thespectral resolution is limited, since the cameras are typically true-colour or false-colourcameras. The data quality depends on the efforts used in the processing, but limitations areintroduced also by available software.The advantage compared to aerial photography is that the data is readily in digital form and itcan be interpreted almost real-time. The disadvantage is narrower opening angle of the sensorrequiring more flight lines to be flown. The advantage compared to very high-resolutionsatellite data is the flexibility in data acquisition and cost-effectiveness. The disadvantage iscomplicated data processing. Digital camera data is expected to become more useful in thefuture as metric digital cameras with wider opening angles are constructed. The dataprocessing methods are improving as well.Acquisition and processing of the airborne SAR dataThe airborne radar flight was done very late when considering the project schedule. Thereasons were several problems with the subcontractors. The flight was in June 2003 and datawas delivered in September 2003. It soon became evident that there were problems with the37
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