UAV-based mapping of nearshore bathymetry over broad areas

In this paper, an unmanned aerial vehicle (UAV)-based method of nearshore bathymetry mapping is developed. The method is based on the bathymetry estimation from a linear dispersion relationship with wave celerity and frequency estimated from a sea-surface video obtained by the UAV. Prior to the bathymetry estimation, image processes including automatic frame rectification are applied to convert the video into sequential orthoimages. The method selectively analyzes the signals originated from swells through filtering processes. Wave celerity of swells is estimated from a time lag of the signals at two locations based on cross-correlation analysis. Bathymetry is estimated along the cross-shore section, then a two-dimensional bathymetry is obtained by repeating this estimation along the shoreline. By applying this method to a video obtained by similar to 5-min hovering at 50 similar to 150-m altitude, bathymetry over an area of similar to 1-km length along the shoreline can be estimated with an accuracy of similar to 0.5 m. The method performs well even on a coast with detached breakwaters without significant decrease in accuracy. Furthermore, we investigate the applicability of the method under various conditions, and the results suggest that the estimation accuracy can be worse under cloudy skies.