Detection of coastal bathymetry using hyperspectral imagery

Amphibious landing operations require environmental information about bathymetry and seabed characteristics of littoral waters so that effective landing strategies can be planned for marine forces traversing the coastal zone. However, in certain situations, ship-based acoustic methods of surveying the environment are not practical. For example, the area of interest may be remotely located, or the survey might be required more urgently than a boat-based survey can be organised. In such situations remote sensing is an attractive alternative using airborne or satellite-based sensors. Recently, a hyperspectral sensing survey using the HyMap system was conducted in Jervis Bay, located about 180km south of Sydney, Australia. HyMap is an airborne hyperspectral remote sensing instrument that collects data in 126 spectral channels from the visible to the shortwave infrared wavelength regions (0.45 to 2.5 um). The bathymetry was retrieved from the sensor data by removing water absorption in depth increments iteratively until the reflectance spectrum is free from absorption effects of water. The result demonstrated that water depths up to 20m can be extracted from the hyperspectral imagery reasonably well compared with that from the conventional hydrographical survey means. Less accuracy is achievable when the water is too deep or too turbid. However, the accuracy can be improved by the use of a few ground truth measurements. The overall results suggest that hyperspectral sensing is likely to be of great value to bathymetric mapping in very shallow waters where traditional boat-based surveys are difficult, dangerous or otherwise may not be possible. With increased developments and launches of satellite-based hyperspectral sensors, there will be more opportunities for obtaining environmental information about bathymetry and seabed characteristics of large coastal water areas cost-effectively and efficiently for both military and civil benefits.