High-Resolution Landslide Monitoring From Digital Imagery

Digital image processing of repeated aerial stereo-photography is highly suitable for landslide monitoring: (1) Automatic extraction of digital terrain models provides changes in surface elevation with a few meters in point density. (2) A special software for automatically measuring horizontal surface displacements with a point density similar to (1) completes the acquisition of the 3D surface kinetics. The accuracy of both techniques approximates the pixel size of the used digital imagery (i.e. approximately 0.1-0.2m for the below presented studies). The techniques are evaluated for two active rock slides in the European Alps: The tongue of Aletsch glacier, the largest glacier in the Alps, lost about 300 m in ice thickness since the Little Ice Age (LIA). The related relief of the adjacent slopes destabilized a mass of ~ 10⁶ - 10⁷ m³ ( ~ 3 ṡ 10⁵ m² area). Surface displacements of up to 2 m horizontally between 1976 and 1995 were measured. Vertical changes of a similar order of magnitude indicated lowering in the upper part of the slide area and corresponding uplift in the lower part. Measurements of higher temporal resolution show a general increase in activity since approximately 1985. Similar to the above Aletsch rock slide, also the shrinkage of Findelen glacier by about 100 m vertically since the LIA caused de-stabilization of a slope section. Between 1985-1999 the mass of ~ 10⁷ m³ ( ~ 2 ṡ 10⁵ m² area) displaced with up to 0.4 m yr^-1 horizontally and settled by a similar amount. Thereby, step-like transverse sub-sections of different settlement speed could be observed. However, the rock slide showed an astonishing coherent deformation during the observational period pointing to a deep-seated shear horizon. Both presented high-resolution studies revealed marked differences in overall dynamic behavior of the sliding masses, although similar processes of de-stabilization are present.