MULTI-TEMPORAL SATELLITE IMAGES WITH BATHYMETRY CORRECTION FOR MAPPING AND ASSESSING SEAGRASS BED CHANGES IN DONGSHA ATOLL

摘要

The Dongsha Atoll is the first ocean national park in Taiwan. It is a 400 kilometer-square atoll with coral reef ecosystems and high biodiversity. Because it has only one small island in the western of atoll and has not opened to public, there are very few human activities and nature is well preserved. To survey this large area is not an easy task, and remote sensing technique provides an efficient and economic approach to survey this area. The multispectral satellite images are useful for assessing sea bottom materials. With multi-temporal images, the changes can also be tracked. A huge event has been observed in summer 2014, more than 25 kilometer-square of seagrass in northern atoll disappear within three months, and the habitat changes from seagrass to sand and coral reef debris. In this study, satellite images with finer temporal resolution will be analyzed to track the speed of changes. However to classify bottom material directly from spectral information has some difficulties, because spectral information is also mixed with other factors, including water absorption coefficient and water depth. In this research, sun glint and water depth correction will be conducted to remove the effects from atmosphere and bathymetry. And image classification for seagrass is followed with multi-source satellite images to track the disappearance and recovery of the seagrass area. The satellite images for experiments include FORMOSAT-2, SPOT-6 and LandSat-8.