Lauren Hintenlang: Nadir and oblique imaging to observe intertidal oyster populations using UAS photogrammetry

Nadir and oblique imaging to observe intertidal oyster populations using UAS photogrammetry

Lauren Hintenlang, Vincent Lecours and Andrew Kane
University of Florida, USA

Challenges in accessing declining, yet essential intertidal oyster habitat and a lack of resources to systematically monitor this fishery-independent habitat have precluded historical data collections to inform resource restoration plans. However, unoccupied aerial systems (UAS) offer a viable approach to overcoming intertidal restoration monitoring barriers. With a variety of platforms and payloads available, they can be customized to fit niche restoration and monitoring needs. For example, while fixed wing UAS are often limited to capturing habitats from above (at nadir), multi-rotor UAS can hover at low altitude to capture habitats from oblique angles. Structure from Motion photogrammetry can then be used to provide accurate orthomosaics, which can be rendered into centimeter-scale 3D models of habitat structure. In this project, five oyster clusters, acting as proxies for future subsampled reefs, were manually imaged at nadir in a transect pattern, akin to what a fixed-wing UAS would capture, and in a 360⁰ pattern, simulating multi-rotor UAS. The images were then imported into Agisoft Metashape to render 3D texturized habitat models. Abundance counts were subsequently performed on the models and physical clusters. While both oblique and nadir methods underestimated the total number of oysters present by 65 and 72.5%, respectively, they did so with notable consistency, achieving standard deviations of 12 and 5%. The mean percent error for estimations made by oblique and nadir methods were -1.7% and -1.8%, respectively, indicating that the methods may be comparable. Such results have important impacts on ongoing studies that extrapolate subsampled data to larger, landscape habitat.