Bathymetry mapping by inversion method using hyperspectral sensor

Water depth information is extremely important for safe marine navigation and offshore security. Currently, it is possible to acquire bathymetric information effectively with multi-beam echo sounder measurements and airborne laser bathymetry scanners. However, Japan is an insular country surrounded by open ocean, and a low-cost technique is needed for the spatial acquisition of shallow-water depths. Experiments with satellite-derived bathymetry using multispectral satellites have been underway since the 1970 s. Their disadvantage is that they always require a field survey. However, the inversion method using hyperspectral data can simultaneously estimate bottom reflectance, water optical properties, and depths without a field survey. This study aims to research and develop a spatially derived water-depth method using hyperspectral data. Using airborne hyperspectral data and field survey data from Yamada Bay, Iwate Prefecture, we investigate adding a bottom index algorithm for classification of benthic type classification into the inversion method processing. We then apply the method to airborne hyperspectral data of Aka-jima Island, Okinawa Prefecture, and demonstrate its versatility with different benthic conditions and optical properties. A comparison to field survey data in Yamada Bay revealed a tendency for estimated depths to be deeper than field survey values, causing a slightly higher RMSE of 3.3 m. On the other hand, in Aka-jima Island, the range of estimated depths was in good agreement with field survey data, with a reduced RMSE of 1.3 m. Also, comparing estimated bathymetry maps to ALB data for Yamada Bay and an M-7000 digital bathymetric chart for Aka-jima Island, we found trends matched except in areas affected by macro alga and rock reefs shadows in Yamada Bay, or by cloud shadows in Aka-jima Island.