Influences to in situ reflectance measurements in shallow water

Abstract

In situ reflectance measurements taken under water may provide useful information about spectral discrimination of shallow water habitats. Often used as input in physical based inversion models or in supervised classification techniques of hyperspectral images, under water spectral libraries are essential to extract these fingerprints of various benthic substrate types. The advantage of reflectance measurements carried out under water is low attenuation of the small water column between substrate and sensor, but the disadvantage is the increased sensitivity to changing illumination, sun-angle, wave action and varying distance to the substrate. It often can lead to exclusion of many valuable spectral readings and/or time intensive statistical corrections. Transferability to other shallow water systems or comparison with other spectral libraries can thus become difficult. A physical understanding of various factors influencing bottom albedo is required. Radiance and irradiance reflectance spectra of homogeneous freshwater macrophytes and sand floor measured with a submersible spectroradiometer in 2003 and 2004 at two different lakes were used to investigate these influencing factors. The aim of this paper is to provide better understanding spectral variability and achieve a better comparison between spectral libraries making usage of such databases more reliable.