On using airborne optical vertical polarisation to remove sea surface reflectance for enhanced visualisation of seagrass and other benthos.

Abstract

Mapping of marine benthic flora using remote sensing techniques has, over the past decade, been used to locate environmentally stressed areas in the South Australian marine environment. These studies used panchromatic/colour aerial photography and/or medium resolution multispectral satellite imagery to create a time series showing location and rate of seagrass loss. While successful within their project parameters, these studies were limited by conditions at time of image capture, such as sun-glare, wave action and low contrast in deeper waters due to absorption and scattering. This research thesis reports the successful use of polarisation on the capture of visible and near infra-red optical imagery as a method to minimise these limiting factors. Two experimental test flights were undertaken using commercial off-the-shelf digital cameras mounted in the camera port of a light aircraft. The first flight compared vertical polarisation using co-mounted visible and infra-red cameras. The second flight compared vertical and horizontal polarisation using co-mounted identical visible spectrum cameras. The main finding of this series of airborne polarisation experiments is that sea surface reflection is removed by using vertically polarised filters at, and around, the Brewster angle of 53 degrees off nadir, especially when viewing sunwards. The effect is the same in the visible and infra-red parts of the spectrum. This reflection includes sky reflectance, lambertian sun glare, reflection due to wave action, and turbulence, but not direct solar specular reflection. Vertical polarisation filters improve the imaging of benthic flora compared to horizontally polarised imagery and, by extension, non-polarised imagery. The successful use of polarisation to remove surface reflectance over water is limited to imagery captured at or near the Brewster angle. By using successive overlapping frames this can be achieved, as shown by the experiments. Ideally all of the image should be at the Brewster angle. A conceptual design for a conical optical scanner which builds wide-swath imagery where each pixel in each band is solely a record of the vertically polarised signal at the Brewster Angle is presented as a result of these experiments.