Parametrization And Preprocessing Of Ocean Color Image Data For Studies Of Coastal Oceanography

Abstract

ABSTRACT Methods are presented for processing ocean color image data into forms suitable for quantitative oceanographic studies. A radiative transfer technique is outlined for determining approximate corrections for atmospheric transmission. atmospheric path radiance. sun glitter. and skylight reflection from the sea surface as functions of scan angle. The radiometric accuracy of these corrections is perhaps no better than ± 20%. but they nevertheless serve to make an overall relative correction for the systematic scan-angle dependent radiance variations with which these effects badly mask the ocean color pattern in images. Further processing methods include removal of geometric scan distortions from each image. and geographic registration (accurate to 0.5 km. or a single pixel) using 'visible landmarks. Work in progress to account for effects of bottom reflectance in ocean color spectrum analyses is reviewed. Experimental results with ocean color spectra from different world locations suggest that it may be possible to standardize the parametrization of ocean color spectra with characteristic vectors. INTRODUCTION Ocean color images obtained from space with scanning radiometers have several potential uses for studying and monitoring oceanic phenomena. Some oft-cited examples include making maps of sediment transport in estuaries and river plumes. monitoring the dispersal of water pollutants. making maps of phytoplankton (chlorophyll) distributions in coastal waters. watching for "red tide" outbreaks. and tracking the drift of dredge-spoil from offshore mining sites. The utility of ocean color imagery has been partially realized with LANDSAT Multi-Spectral Scanner (MSS) data. even though the MSS was not designed for water color measurements (its radiometric gains are too low and its spectral bandwidths are too wide). Better ocean color image data will become available in 1978. when the Coastal Zone Color Scanner (CZCS) flies on the Nimbus-G satellite. The CZCS will have four narrow-band channels between 400 and 700 nm, a broad-band channel in the 7001100 nm region, and a thermal infrared channel in the 10–12 µm atmospheric window. Radiometric gains on CZCS will resolve the low radiance levels emanating from the sea and will saturate over most land scenes. Finally, Nimbus/CZCS coverage of a given scene will be repeated approximately every 36 hours. compared to 18 days for LANDSAT/MSS. In order to simulate CZCS data for algorithm and software development and for pre-launch experiments. a 10channel ocean color scanner (OCS) is currently operated aboard a NASA U-2 aircraft. Multispectral radiometric ocean color measurements with. e.g. the OCS. are multidimensional data. and therefore. somewhat awkward to handle, display and interpret.