Author(s): Martin Detert; Volker Weitbrecht
Keywords: Computer vision; field studies; flow visualization and imaging; Particle Image Velocimetry (PIV); streams and rivers; unmanned aerial vehicle (UAV); velocity measurements
Abstract: This Technical Note proves the applicability of a low-cost airborne velocimetry system to measure large-scale surface velocity fields. The measurement equipment consists of an ultra-light action-cam and a ready-to-fly low-cost quadrocopter. Video recordings were performed from heights between 45–74 m covering a total reach length of 310 m, while spruce chips were added as tracer particles. Each lens-corrected frame was automatically ortho-rectified to riparian ground reference points. The positional error of each point was computed to be within 0.17–0.39 m, so that the magnitude of the related descaling error was below ±2%, and the error of apparent ground velocity is approximately 0.03 m s−1. These values describe the uncertainty added to the subsequently calculated particle image velocity field. The final raster resolution was 1.0×1.0 m2 with 50% overlap. A comparison with the velocity profiles measured by a 3D acoustic Doppler current profiler indicates that the proposed new type of velocimetry system is capable of measuring with relatively high accuracy.