A low-cost airborne velocimetry system: proof of concept

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 0.03ms(-1). These values describe the uncertainty added to the subsequently calculated particle image velocity field. The final raster resolution was 1.0x1.0 m(2) 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.