Abstract
Abstract. Historical aerial images represent a source of information of great value for glacier monitoring, as they cover the area of interest at a well-defined epoch and allow for visual interpretation and metric analysis. Typically, the aerial images are used to produce orthophotos and manually digitize the perimeters of the glaciers for analysis of the surface extent of the glaciers, while the extraction of height information is more challenging due to data quality and characteristics. This article discusses the potential of historical aerial images for glacier modelling. More specifically, it analyses the impact of their coverage, radiometric- and geometric accuracy, state of preservation and completeness on the photogrammetric workflow. The data set used consists of scans of 300 (analog) aerial images acquired between August and October 1954 by the U.S. Air Force with a Fairchild KF7660 camera over the entire Province of Trento. For the modelling of the glaciers, different techniques such as manual stereoscopic measurement and dense image matching were tested on sample glaciers and the results were analysed in detail. Due to local radiometric saturation in a large part of the glacial surfaces and other disturbances affecting the historical images (e.g. scratches, scanning errors, dark shadows), dense image matching did not produce any valuable results, and stereo plotting could be used only on images (or image parts) with acceptable quality. The derived Digital Terrain Models (DTMs) were compared with a reference DTM obtained with dense image matching from digital aerial images acquired in September 2015 with an UltraCam Eagle sensor, and, for some glaciers, to a DTM obtained with dense image matching from scanned aerial images acquired in September 1983 with a RC30 analog camera. The differences between 1954 and 2015 DTMs showed values up to 70–80 m in height and a behaviour that is confirmed by the models employed by the glaciology experts in Trento.
Highlights
Historical aerial images represent a source of information of great value for glacier monitoring, as they cover the area of interest at a well-defined epoch and allow for visual interpretation and metric analysis
The technological advances in digital photogrammetry in the last decade have allowed the production of Digital Surface Models (DSMs, equivalent to Digital Terrain Models (DTMs) on glaciers as there are no vegetation and no man-made objects) from digital aerial images with Dense Image Matching (DIM)
To evaluate the 2D/3D information of glaciers extracted from the 1954 images, the glacier outlines and terrain models were compared to the available data from other years, and the changes were matched to the prediction models used by the glaciology experts
Summary
Historical aerial images represent a source of information of great value for glacier monitoring, as they cover the area of interest at a well-defined epoch and allow for visual interpretation and metric analysis. The technological advances in digital photogrammetry in the last decade have allowed the production of Digital Surface Models (DSMs, equivalent to DTMs on glaciers as there are no vegetation and no man-made objects) from digital aerial images with Dense Image Matching (DIM) These techniques have proved to be successful on glaciated areas, if modern digital cameras and suitable overlaps are used for the image acquisition (Legat et al, 2016, Mölg and Bloch, 2017); they have the great advantages of the lower cost compared to other technologies (i.e. airborne laser scanning) in case of new acquisitions, and the possibility to be employed on archived images with suitable overlaps and image quality. Historical aerial images represent one of the oldest data sources covering glaciers extensively They are of great value for glacier monitoring and often provide the basis for area-, length-, volume- and mass change analysis. The coverage is suitable for aerial triangulation and orthophoto production in flat, hilly and mountain areas
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