Uncertainties in bathymetric retrievals

Abstract

The sensitivity of the remotely-sensed water reflectance to variations in water depth and bottom albedo is investigated using a semianalytical model and the radiative transfer numerical model HYDROLIGHT. Properties of the water column are taken from published observations made on 14 Feb 2001 in the coastal waters of Moreton Bay (southeast Queensland). The sensitivities of reflectance to depth and albedo are computed using partial derivatives derived from the semianalytical model, and with finite differences using the numerical model. These are compared for various depths and bottom conditions, and also to the instrumental resolution of the Hyperion satellite remote sensing system. Sensitivity to albedo is found to be greater than sensitivity to depth under some situations making the depth signal in reflectance ambiguous. These sensitivities are then applied to bathymetry algorithms to produce error bars for depth retrievals under variable bottom albedo conditions. This is done for both the standard algorithm that uses reflectances at 550 and 650 nm corrected for optically deep water reflectance, and a blue/green ratio algorithm intended for use over low albedo areas. In the standard algorithm variations of 10% in albedo produce errors in depth estimates of approximately 0.05m over sandy bottoms which is small compared with the 2m depth resolution required to reach the bottom in the Moreton Bay experiment. The blue/green ratio algorithm showed no skill for bathymetric mapping with this dataset.