Surf Zone Characterization Using a Small Quadcopter: Technical Issues and Procedures

We explore the potential for using a small unmanned aerial vehicle (UAV) quadcopter to collect long-dwell imagery of the nearshore from which important measurements can be made at low cost and with flexibility. This paper extends the existing topographic imaging approaches that rely on having plentiful ground control spread across the image, to the nearshore case where the bulk of the image is water with no control point and vehicle metadata must be used. The UAV autopilot was found to be capable of excellent station-keeping with the positional errors of 0.20 and 0.53 m (horizontal and vertical) and the viewing angle errors of 0.25° (tilt and roll) and 0.38° (azimuth). The ground position of imaged objects could be found with 0.21-m accuracy. Metadata returned by the UAV on camera position was accurate at 5 m, and the camera roll could be assumed to be 0°, reducing the ground control requirements to two, or even one location. Even under this extreme simplification, ground position errors averaged only 10 m but were worse for cases when only control points near to the UAV were used. A model for the visual contrast of waves when viewed from different angles found that large tilts are important but, in contrast to theory, that there was little dependence on the viewing azimuthal angle. Derived Argus products agreed well with the same products collected using a traditional fixed Argus station. UAVs appear to be a very promising alternate to fixed camera systems if limited duration sampling is adequate.