Hyperspectral Coral Reef Classification of Bonaire
In order to update the 1985 atlas of Bonaire’s coral reefs (Van Duyl, 1985), a hyperspectral mapping campaign was performed in October 2013 using the Wageningen UR Hyperspectral Mapping System (HYMSY) with 101 spectral channels. In June 2016, with the help of STINAPA Bonaire, Sander Mücher and Erik Meesters were able to perform an extensive diving campaign to collect in-situ information in 20 transects perpendicular to the coastline across the western coast. Detailed photographs of the sea bottom were taken by diving to a depth of 20-30 meters, then progressing shallower towards the coast. Photographs were geotagged by another person snorkelling exactly above the diver with a GPS in a waterproof box and making overview pictures of the sea bottom as well.
This in-situ information was used to interpret the hyperspectral imagery made by the HYMSY camera. In order to interpret the data more consistently, the hyperspectral data were corrected for the water depth into at-ground-reflectance factor units. A bathymetric model was used for the calibration of the hyperspectral imagery based on a former field campaign by measuring water depth at specific locations along the western coast. The final bathymetric model was based on extrapolation of the terrestrial digital elevation model by fitting the DEM with additional in-situ bathymetric measurements at sea. A more detailed bathymetric model may improve the calibration of the hyperspectral data with a 1-meter spatial resolution.
Due to the limited penetration of green and red light through the water, it was decided to use only the first 15 hyperspectral bands in the violet-blue -cyan till green range (Band 1 = 450.0 nm to Band 15 = 520.0 nm). This means that spectral measurements can be made to a maximum depth of 20 to 30 meters. The hyperspectral reef classification defines 12 classes including sand, pavement, rubble, soft and hard corals, and various mixtures of these at a pixel level of 1-meter spatial resolution. Due to their spectral similarity, corals and algae remain difficult to separate.