Mapping seagrass and seaweed bed habitats is essential for the implementation of sustainable development in coastal waters. In this study, we attempted to create orthomosaic maps from drone images of seagrass/seaweed beds taken of the Takenoura area in Onagawa Bay, Miyagi Prefecture, Japan. We conducted 8 surveys in 2019 and 2020 and successfully constructed a total of five orthomosaic maps. On these maps, seagrass/seaweed beds were either clearly visible (3 surveys) or visible but not clear (2 surveys), based on visual inspection. We failed to construct acceptable maps of 3 surveys, and in one case, could not create even a fraction of a composite map. We found that, to be able to create successful maps, it was necessary to establish a wide enough survey area to include fixed ground features such as land or rock outcrops, and to have sufficient overlap between drone images. In this study, the overlap ratio was set at 70 % on both the front and the side. Our results also suggest that drone surveys be conducted: (1) during low tide so that the distance between the sea surface and the seagrass/seaweed beds (seafloor) is shorter, and (2) under conditions of low turbidity so that the seagrass/seaweed beds can be seen through the sea surface. Attaching a neutral density and circularly polarized luminescence filter was also considered effective to reduce sunlight reflection on the sea surface. Although drone surveys have been reported to be effective in shallow waters in which the seafloor is clearly visible, in our study, orthomosaic maps could be created even in deep waters in which underwater features could be seen. Finally, underwater surveys by divers were conducted for ground truthing. Habitats of both seagrasses (Zostera marina and Zostera caulescens) and seaweeds (Sargassum micracanthum and Sargassum siliquastrum) were confirmed.
