Sun glint correction of airborne AISA images for mapping shallow-water benthos

Abstract

Airborne sensors have higher spatial and spectral resolution than satellite sensors, providing greater accuracy in benthic habitat mapping in case of high spatial heterogeneity. Nevertheless, the effect of wave-induced sun glint may obscure the radiance originating from within the water. Such glint is particularly noticeable due to the high spatial resolution of the sensor and may impede mapping of benthic features. This paper describes the application of the sun glint correction schemes on to airborne hyperspectral AISA measurements acquired on the area of the West-Estonian archipelago during the campaign in July 2006. Currently proposed sun glint removal procedures assume zero water leaving signal in near infrared part of spectrum. This assumption is not true in waters less than about 2 m deep where part of the water leaving signal is originated from the bottom. As a result the shallow water pixels are overcorrected during glint removal procedure and the shapes of reflectance spectra are distorted. This has serious implications on shallow water bottom classification results, especially if spectral libraries of in situ measured or modelled reflectance spectra are used in classification of remote sensing imagery. Therefore, it is important to preserve spectral signatures of these areas if sun glint removal is necessary. We propose an alternative sun glint removal procedure where the amount of glint in each pixel is estimated from the depth of oxygen absorption feature at 760 nm relative to a baseline. The new method removes sun glint successfully and at the same time preserves the shape and magnitude of shallow water reflectance spectra.