Streambank topography: an accuracy assessment of UAV-based and traditional 3D reconstructions

Abstract

Highly accurate digital surface models are an essential part of change-over-time analyses for monitoring erosion processes. Streambank topography presents a unique challenge for surface mapping due to dense riparian vegetation, canopy cover, and rapidly changing elevation values. The spatial heterogeneity of stream corridors has made the calculation of streambank erosion across larger spatial extents difficult. Contemporary technologies such as terrestrial laser scanners (TLS) and unmanned aerial vehicles (UAVs) offer new approaches for streambank topography mapping at very high spatial resolutions across varying spatial extents. To evaluate the accuracy of different technologies for streambank topography mapping, we compared streambank surface models derived via a UAV using structure-from-motion and from traditional aerial photogrammetry (i.e. Southwestern Ontario Orthoimagery Project; SWOOP) to that of a TLS benchmark across seven streambank segments. Additional comparisons were made for 22 manually measured stream transects to that of a TLS benchmark. Compared to our benchmark, the UAV-derived streambank surface model was the most accurate with an average root-mean-square-error of 0.104 m. Errors in the UAV surface model were correlated with georeferencing error. The UAV had an average 52% success rate for reconstructing the streambank topography across all field campaigns and was able to map up to 2037 m of streambank in one hour. The streambank surface model derived from traditional aerial photogrammetry and manual transect measurements had average root-mean-square-errors of 0.238 m and 0.274 m respectively. Both aerially-derived surface models tended to over measure elevation values compared to the TLS, whereas manual transect measurements consistently under measured elevation.