Georeferencing of UAV imagery for nearshore bathymetry retrieval

Abstract

In dynamic coastal areas where rapid changes in the seabed can have a significant impact on day–to–day operations, as is the case of Figueira da Foz harbor inlet, it is important to have updated knowledge of the bathymetry. As in energetic wave conditions it is not possible to carry out traditional bathymetric surveys, this work proposes a methodology that allows, using UAVs, to estimate the bathymetry through bathymetric inversion methods. On two different occasions, flights were carried out with a commercial UAV to obtain local bathymetry at Figueira da Foz harbor inlet. Since this location is far from the coast, there is not a minimum amount of land to use ground control points that allow georeferencing the acquired images in the traditional way with the necessary spatial resolution. Therefore, an automatic image georeferencing methodology was developed and validated in this work. A spectral analysis is used to retrieve both wave periods and wavelengths, respectively, by evaluating the positions of the wave crests in both time and spatial domains. From this information, the water depths are estimated using the linear dispersion relation and the results are validated with field survey data. The automatic georeferencing methodology is shown to be capable of placing the image in the correct location by overlaying on a generic basemap. The spectral analysis showed positive results in estimating wavelengths and wave periods. However, the quality of bathymetric inversion in areas where morphological structures exist (sandbars), presents results with different accuracies depending on the wave and tide characteristics observed each day.