An Application of Low-altitude Remote Sensing Using a Vessel-towed Balloon for Monitoring Jellyfish Patchiness in Coastal Waters

抄録

Low-altitude remote sensing, using a digital camera suspended from a vessel-towed balloon, was adopted to monitor jellyfish patchiness in coastal waters. The balloon, towed by a rope from a research vessel at about 200m altitude, takes photographs of the sea surface while the vessel moves through jellyfish patchiness. This balloon photography satisfies two essential technical requirements for surveying relatively small objects drifting in coastal waters. First, the photographs must be converted into images to which our sight line is perpendicular, because they are generally taken at oblique angles to the horizontal plane; otherwise positions and areas of jellyfish patchiness cannot be computed accurately. Hence, a projective transformation was applied for this geo-referencing, using polystyrene form boards with a GPS receiver for providing reference positions. In addition, to enhance the reference points sufficiently, GPS data recorded on the vessel were augmented in a single photograph by superimposing multiple photographs taken within a short time interval. Second, jellyfish patchiness must be extracted from the photographs efficiently and accurately. The color difference between the jellyfish and the background color of the ocean permitted extraction of the patchiness from photographs on a color space based on hue, saturation, and intensity components. This color space was useful in removing the strong levels of reflected sunlight at the sea surface by lowering the intensity of colors artificially, under the assumption that any white color was due to reflected sunlight.