Towards precise drone-based measurement of elevation change in permafrost terrain experiencing thaw and thermokarst

Abstract

Measuring ground elevation changes plays a crucial role in several environmental applications. For instance, permafrost soils undergo seasonal active layer freezing and thawing that causes cyclic elevation changes. Permafrost thaw can result in unidirectional ground subsidence, which may be gradual and uniform, or rapid and irregular in the case of thermokarst landforms such as slumps and degrading ice-wedges. Photogrammetric drone surveys have effectively characterized large (> 0.1 m) ground elevation changes resulting from thermokarst, yet many permafrost processes of interest lead to more subtle elevation changes. In this study, we assessed various drone-based surveying strategies for their precision to measure smaller (< 0.1 m) ground elevation changes to better characterize permafrost-driven surface dynamics. The strategies were compared by examining the short-term reproducibility of modeled elevation for 76 bare ground targets, derived from six repeat drone surveys captured under variable illumination. We found that the Phantom 4 RTK drone using direct georeferencing, combined with one fixed ground control point, could reproduce elevations with a mean absolute deviation of 0.6 cm, suggesting a minimum level of change detection of 1.4 cm at 95% confidence. Drone-based methods for measuring permafrost elevation changes should be complementary to in situ and satellite-based (e.g. differential interferometric Synthetic Aperture Radar) approaches.