Mapping of selected coral reefs in Southern, Sri Lanka using remote sensing methods
T.W.S. Warnasuriya , P.B.T. Pradeep Kumara, N. Alahacoon
Most parts of the Southern coastal belt of Sri Lanka are fringed with shallow water coral reefs of high economic, ecological and socioeconomic attributes. Polhena, Madiha and Dondra are some of the places where the isolated coral reefs are located in this region. Descriptive details such as the extent, reef margins, and coral compositions of these reefs are lacking due to the difficulties of accessibility, harsh conditions and the lacking of trained expertise for reef mapping. Remote sensing can be used as a tool to identify the reef locations, spatial distribution, bottom nature, present status of the reef area for their effective management and conservation. In this research, bathymetry maps were produced using ‘Depth of Penetration’ method for Landsat7 ETM+ satellite images. Image band 1 (0.45-0.515μm), band2 (0.525-0.605μm), band3 (0.63-0.69μm) and band4 (0.75-0.9μm) have been used as main inputs of bathymetry mapping. Colour composites, classified images and manually digitized images were used to map the spatial pattern of the reefs. The image processing has been carried out with ILWIS 3.3 and ArcGIS 9.3 software. Created maps were validated using ground truth data which were taken during the field survey. The Pearson correlation coefficient (r=0.921) indicates that there is a high correlation between true depth and the calculated depth from the map. The standard error of the estimate is 0.85. According to the created GIS maps, maximum depth (1.83±0.79m) was recorded in Dondra reef while lowest depth (1.46±1.23 m) was recorded in Polhena reef. Depth of the madiha reef was 1.81±0.99 m. Highest reef area represented by the Polhena reef (240300 m2) while lowest area represented by the Madiha reef (51300 m2). Area of the Dondra reef was 85500 m2. Band 4 is the best for shallow (<4m) reef mapping and high resolution images are ideal for more detailed habitat mapping.
Keywords: Coral Reefs, Remote sensing, Landsat7 ETM+, Bathymetry, Sri Lanka