Structure-from-Motion-Derived Digital Surface Models from Historical Aerial Photographs: A New 3D Application for Coastal Dune Monitoring

Recent advances in structure-from-motion (SfM) techniques have proliferated the use of unmanned aerial vehicles (UAVs) in the monitoring of coastal landform changes, particularly when applied in the reconstruction of 3D surface models from historical aerial photographs. Here, we explore a number of depth map filtering and point cloud cleaning methods using the commercial software Agisoft Metashape Pro to determine the optimal methodology to build reliable digital surface models (DSMs). Twelve different aerial photography-derived DSMs are validated and compared against light detection and ranging (LiDAR)- and UAV-derived DSMs of a vegetated coastal dune system that has undergone several decades of coastline retreat. The different studied methods showed an average vertical error (root mean square error, RMSE) of approximately 1 m, with the best method resulting in an error value of 0.93 m. In our case, the best method resulted from the removal of confidence values in the range of 0–3 from the dense point cloud (DPC), with no filter applied to the depth maps. Differences among the methods examined were associated with the reconstruction of the dune slipface. The application of the modern SfM methodology to the analysis of historical aerial (vertical) photography is a novel (and reliable) new approach that can be used to better quantify coastal dune volume changes. DSMs derived from suitable historical aerial photographs, therefore, represent dependable sources of 3D data that can be used to better analyse long-term geomorphic changes in coastal dune areas that have undergone retreat

The Application of X-Band Radar for Characterisation of Nearshore Dynamics on a Mixed Sand and Gravel Beach

Remote sensing using X-band radar allows the estimation of wave parameters, near surface currents and the underlying bathymetry. This paper explores the use of radar to derive nearshore bathymetry at a complex site, at Thorpeness in Suffolk, UK. The site has a history of sporadic and focused erosion events along the beach frontage and as part of the X-Com project (X-band Radar and Evidence-Based Coastal Management Decisions) a radar system was deployed with the aim of further understanding the complex nearshore sediment processes influencing erosion. Initially, the bathymetric variation at the site is quantified through analysis of current and historic multibeam surveys. These indicate depth changes approaching 3 m. Subsequently, validation of the radar data against concurrent multibeam survey data has been undertaken. Results show that the radar derived bathymetry has a precision of ±1m at the site, with the largest errors being associated with areas of more complex bathymetry and where wave data quality was less suitable for analysis by the X-band radar bathymetry algorithms. It is concluded that although the accuracy of radar-derived bathymetry is lower than traditional multibeam survey, the low cost for high temporal coverage can be utilised for long-term monitoring of coastal sites where a cost-effective means of quantifying large-scale bathymetric changes is required