REMOTE-SENSED MAPPING OF SEAGRASS DISTRIBUTION IN PALK BAY, SRI LANKA, USING HIGH SPATIAL RESOLUTION WORLDVIEW-2 SATELLITE DATA

摘要

This study incorporates field observations and high spatial resolution WV-2 imagery processing techniques to provide an assessment of shallow coastal marine seagrass beds in Palk Bay, North-Westem coast of Sri Lanka. The main objective of this study is to influence decision making and coastal planning in Sri Lanka with the increased knowledge on the seagrass habitats in Palk Bay with special reference to Dugong conservation. Field observation were conducted in once a month during 2015 to 2016 and methods included free diving, monitoring transect lines, quantify quadrats, and underwater photography techniques. Common species encountered in study areas were Enhalus acoroides, Cymodocea rotundata, Cymodocea serrulata and Halodule pinifolia. Cloud free WV-2 satellite imageries of 15~(th) June 2015 and 11~(th) February 2016 were selected as remote sensing data sources. After image pre-processing, supervised image classifications were performed using maximum likelihood, minimum distance to means, and spectral angle mapper methods to compare relative accuracies in mapping seagrass coverage. The maximum likelihood classification produced the highest overall accuracy of 94%. The spectral angle mapper yielded the lowest accuracy due to the predominant influence of water-column optical properties on the apparent spectral characteristics of seagrass and sand bottom. The results achieved by our classification methodology were validated with visual interpretation and field data. The combination of in-situ data and three classification methods resulted in highly accurate classification outcomes that showed the distribution patterns of seagrass of the study area. Based on the results, we conclude that eight-band high resolution multispectral WV-2 satellite imagery has great potential for mapping and monitoring seagrass beds in shallow coastal waters with large-scale coverage. Thus, the primary results of this study provide useful baseline information that is necessary for marine-conservation strategic planning incorporated to protecting feeding grounds of dugongs around the North-Western coast of Sri Lanka.