Photogrammetric reconstruction of homogenous snow surfaces in alpine terrain applying near-infrared UAS imagery

In this study, we investigate the applicability and performance of UAS-based structure-from-motion SfM photogrammetry on very homogenous snow surfaces, under suboptimal illumination conditions, at two alpine test sites in Tschuggen 2000 m a.s.l. close to Davos, Switzerland, and Lizum 2000 m a.s.l. near Innsbruck, Austria. We discuss the topographic and meteorological challenges for flying UAS missions in high-alpine terrain. Additionally, we compare DSMs calculated from the imagery acquired in the visual VIS, λ = 400–700 nm and near-infrared NIR, λ = 700–830 nm parts of the electromagnetic spectrum. We evaluate the resulting DSMs qualitatively and quantitatively by applying: a differential Global Navigation Satellite System GNSS measurements at the Swiss test site, with an expected accuracy better than 0.1 m within x, y, and z directions; b terrestrial laser scanning TLS at the Austrian test site, with an expected accuracy of ± 0.025 m 1σ, along with a distance-dependent error. The results of this study reveal the potential and limitations of UAS-based SfM photogrammetry for applications on snow-covered, alpine terrain in general and in particular the benefit of NIR imagery on the accuracy and precision of the results.