Coral Reef Benthos Classification Using Data from a Short-Range Multispectral Sensor

A five-band short-range multispectral sensor (MicaSense RedEdge-M) was adapted to an underwater housing and used to obtain data from coral reef benthos. Artificial illumination was required to obtain data from most of the spectral range of the sensor; the optimal distance for obtaining these data was 0.5 m, from the sensor to the bottom. Multispectral orthomosaics were developed using structure-from-motion software; these have the advantage of producing ultra-high spatial resolution (down to 0.4 × 0.4 mm/pixel) images over larger areas. Pixel-based supervised classification of a multispectral (R, G, B, RE bands) orthomosaic accurately discriminated among different benthic components; classification schemes defined 9 to 14 different benthic components such as brown algae, green algae, sponges, crustose coralline algae, and different coral species with high accuracy (up to 84% overall accuracy, and 0.83 for Kappa and Tau coefficients). The least useful band acquired by the camera for this underwater application was the near-infrared (820–860 nm) associated with its rapid absorption in the water column. Further testing is required to explore possible applications of these multispectral orthomosaics, including the assessment of the health of coral colonies, as well as the automation of their processing.