Surface currents retrieved from airborne video

Within this paper, we describe a methodology to retrieve very high-resolution ocean surface currents from short video sequences of water surface waves. The video sequences were acquired in the range of visible light with a small, nadir looking 4k video camera attached to an off the shelf quadcopter. The quadcopter is equipped with an actively controlled gimbal for stabilization of the video camera. Furthermore, the quadcopter is equipped with a logger that records all the flight information, which is used to geocode the video data. In a first step, the video data are corrected for the distortion of the lens and are then geocoded to a rectilinear coordinate system at water level. The resulting video data allow for measuring surface wave properties such as wave direction and length as well as phase velocity. These properties of the waves enable to estimate the surface current vector, which results from the difference of the observed phase velocity to that given by the linear dispersion relation of surface gravity waves. For testing purposes, the method was applied to video data, which were collected over the Elbe River in vicinity of a weir. The resulting surface current field nicely represents the surface flow and indicates the overall applicability of the method.