Research On Coastal Bathymetry Algorithm Based On Time Series Images Collected By Small UAVs

In the field of coastal engineering,the measurement of shallow water bathymetry has always been a big challenge.Scientists have proved that the wave speed can be estimated by observing waves,and then the actual water depth can be obtained by the method of inversion.In this paper,we derive the shallow water dispersion relationship of the linear wave theory based on the assumption that seawater is an ideal fluid,and prove the rationality and feasibility of the water depth inversion algorithm on this basis.We use a small UAV to collect the video data of actual ocean wave,from which we build the data set required by the cBathy water depth inversion algorithm(proposed by Professor Rob holman from Oregon State University in the United States and successfully applied to the Argus Coastal Monitoring Station).Then we implement and evaluate the cBathy algorithm by two orthographic projection methods,one based on the image ground control points and the other based on the camera attitude angle.The main contents of this paper can be summarized as follows:(1)We assume that seawater is ideal fluid,and derive the Euler equation of the ideal fluid from the law of conservation of mass and the law of conservation of momentum.Then we prove that the depth inversion algorithm based on the linear dispersion relation of shallow water is theoretically reasonable and feasible in practice.(2)We go into the principles and implementation process of Complex Empirical Orthogonal Function algorithm and cBathy algorithm,and construct the data set of cBathy algorithm from the video of actual ocean wave.We focus on the realization of cBathy algorithm,calculating the water depth by combining the estimation method of frequency-wavenumber pair and the best-fit method.Then we obtain the complete water depth estimation map.We also analyze the possible problems in the process.(3)We use a small drone equipped with a camera to collect the data of actual ocean wave,and use the camera imaging model to obtain the conversion from the world coordinate system to the camera coordinate system.After the image sampling and the distortion elimination,we calibrate the image using ground control points or markers,then re-project the image into an orthographic projection respectively using the ground control points and camera attitude angle.(4)We use the actual data to test the orthographic projection methods based on ground control points and camera attitude angles that we propose in this paper,and the cBathy algorithm is used to obtain the corresponding water depth maps for comparison.The impact on water depth Estimation of these two methods are studied.According to the results,the result of the method based on ground control points is significantly better than that of the method based on camera attitude angle.Then we come to a conclusion from the results that the accuracy of water depth estimation depends on the accuracy of the orthographic projection.However,the method based on camera attitude angle is still feasible,although it is necessary to provide a higher-precision attitude angle.