The use of computer vision to relocate historical aerial photographs that enhance the understanding of hydrogeomophic changes in Ethiopia

The digitisation of historical aerial photograph archives offers a unique opportunity to analyse long-term environmental changes. One such valuable resource is the archive of 1935-1941 aerial photographs of Ethiopia, one of the largest and oldest collections in Africa, comprising 34,000 images. While a portion of these images has been localized, many remain without known coordinates. To address this, we developed a computer vision approach that combines scale invariant feature transform (SIFT) keypoint matching and nearest-neighbour search, achieving 99% accuracy and 80% recall in matching images. This method increased the localization rate from 40% to approximately 70%, though manual verification and coordinate determination remain necessary. A proof-of-concept further demonstrated the potential of utilizing depth information to localize photographs: by leveraging the spatial proximity of images within the quite erratic flight lines, we significantly reduced the search area. Additionally, we show that 3D scene reconstruction from consecutive images, matched to a digital elevation model using the ICP algorithm, is feasible.

We demonstrate the potential of historical aerial archives for studying long-term environmental change through a case study on river geomorphology. At 70 locations where aerial photographs intersect major unconfined rivers, we analysed key hydrogeomorphological variables to assess river dynamics. By comparing river morphology in 1935-1941 with that on the most recent Google Earth imagery, our results reveal significant morphological changes, including channel widening, gullying, bank erosion, and in-stream sediment accumulation. These findings highlight how a detailed understanding of local river dynamics, derived from historical and modern imagery, can enhance the broader understanding of environmental changes and their impacts on catchment behaviour.

Key words: Aerial Photographs, Environmental Change, Hydrogeomorphology, Environmental Change, River