Using structure-from-motion to create glacier DEMs and orthoimagery from historical terrestrial and oblique aerial imagery

Increased resolution and availability of remote sensing products, and advancements in small-scale aerial drone systems, allows observations of glacial changes at unprecedented levels of detail. Software developments, such as structure-from-motion (SfM), now allow users an easy and efficient method to generate three-dimensional (3D) models and orthoimages from aerial or terrestrial datasets. While these advancements show promise for current and future glacier monitoring, many regions still suffer a lack of observations from earlier time periods. We report on the use of SfM to extract spatial information from various historic imagery sources. We focus on three geographic regions, the European Alps, high Arctic Norway and the Nepal Himalayas. We used terrestrial field photographs from 1896, high oblique aerial photographs from 1936 and aerial handheld photographs from 1978 to generate digital elevation models (DEMs) and orthophotos of the Rhone glacier, BrOggerhalvOya and the lower Khumbu glacier, respectively. Our analysis shows that applying SfM to historic imagery can generate high quality models using only ground control points. Limited camera/orientation information was largely reproduced using self-calibrated model data. Using these data, we calculated mean ground sampling distances across each site which demonstrates the high potential resolution of resulting models. Vertical errors for our models are +/- 5.4m, +/- 5.2m and +/- 3.3m. Differencing shows similar patterns of thinning at lower Rhone (European Alps) and BrOggerhalvOya (Norway) glaciers, which have mean thinning rates of 0.31m a(-1) (1896-2010) to 0.86m a(-1) (1936-2010) respectively. On these clean ice glaciers thinning is highest in the terminus region and decreasing up-glacier. In contrast to these glaciers, uneven topography, exposed ice-cliffs and debris cover on the Khumbu glacier create a highly variable spatial distribution of thinning. The mean thinning rate for the Khumbu study area was found to be 0.54 +/- 0.9m a(-1) (1978-2015). Copyright (c) 2017 John Wiley & Sons, Ltd.