Fluvial and aquatic applications of Structure from Motion photogrammetry and unmanned aerial vehicle/drone technology

Structure from motion (SfM) has seen rapid uptake recently in the fluvial and aquatic sciences. This uptake is not least due to the widespread availability of cheap unmanned aerial vehicles/drones, which help mitigate the challenging terrain and deliver efficient and reproducible and high‐accuracy images and topographical data. These data can have unprecedented spatio‐temporal coverage and includes measurements of fluvial and aquatic topography, hydraulics, geomorphology and habitat quality. SfM data also offer novel quantification of underwater archeology, structures and aquatic organisms. Studies are shifting from proof‐of‐concepts in topographic survey to genuine applications including grain‐size mapping, bathymetric surveys, geomorphological mapping, vegetation mapping, restoration monitoring, habitat classification, geomorphological change detection and sediment transport path delineation. Integrating point cloud analyses and orthophoto mosaics with digital elevation models has been shown to be effective in providing novel process understanding of fluvial and aquatic systems. Underwater and through‐water studies are beginning to overcome problems of accessibility, visibility and image distortion. Archival photographs and video (both above‐ and under‐water) are being reprocessed using a SfM workflow to generate three‐dimensional surfaces and objects from historical surveys, thereby extending the time period over which change can be detected. Recently, a SfM workflow has been developed to model free water surfaces with clear potential for future exploitation in hydraulics, sediment transport and river bed evolution studies. Future applications of SfM could seek to exploit the daily repeat coverage of high‐resolution satellite images but must be mindful of the necessary investment in this development versus the increasing availability and coverage of spaceborne light detection and ranging.