1 M. S. Swaminathan Research Foundation, Taramani, Madras
2 Institute for Ocean Management, Anna University, Madras
Abstract
The launch of Earth Resources Technology Satellite -1 (ERTS-1) in 1972 proved to be of great importance as it gave a new tool of remote sensing to the scientific community. The subsequent launch of the Landsat series of satellites, the French satellite Systeme Probatorie d'Observation de la Terre (SPOT) and the Indian Remote Sensing (IRS) satellites with high spatial resolution increased the potentiality of this new tool for coral reef mapping. Case studies in Gulf of Mannar and Andaman & Nicobar Islands have proved the mapping of reef categories such as fringing reef, patch reef, platform reef with or without central lagoon which are clearly discernible in SPOT and IRS satellite imagery. Also the tidal action and water circulation patterns in reef areas between 2 and 10 km coastal belt of Andaman & Nicobar Islands were mapped using remote sensing data. Case studies in Gulf of Mannar using GIS tool have brought out the changes in coastal configuration due to erosion and accretion and its impact on the adjacent coral reef ecosystem.
Optical Remote Sensing Data
Studies were carried out for mapping the coral reefs in Andaman & Nicobar and Gulf of Mannar islands using optical remote sensing data especially SPOT, Landsat TM and IRS LISS-II. Due to high spatial resolution, all the above sensor data are found to be useful for coral reefs mapping and monitoring. More details on reef categories like fringing reef, platform reef, patch reef and coral pinnacles could be mapped more accurately using SPOT and IRS data by visual analysis. The fringing reefs are adjacent to the coast and are the predominant type bordering the western coast of Andaman & Nicobar islands. Reefs occur in shallow depth (within one to two meters depth) will appear in blue tone and the deep reefs in turquoise blue tone. Tonal variation is the important key element to classify and map shallow and deep reefs using satellite imagery. Validation of coral reef maps by ground trotting has proved that reefs occur up to 13 meters depth in less turbid coastal waters could be demarcated from satellite imagery by visual interpretation. Based on visual interpretation of SPOT and IRS imagery and incorporation of SOI toposheet details, fifty two coral reefs maps on 1:50,000 scale for Andaman & Nicobar and seven maps for Gulf of Mannar were jointly prepared and validated by the Institute for Ocean Management, Anna University and the Space Applications Centre (ISRO).
Digital analysis using VIPS32 functions in VAX 11/780 image analysis system has brought out the utility of visible bands of Landsat TM for mapping coral reefs and other near shore parameters like suspended particulate matters and ocean colour variations due to chlorophyll concentration, etc. Selected image analysis functions like factorial analysis, band rationing, local optimisation, convolution filtering and correspondence analysis were attempted using TM digital data of Gulf of Mannar coral reef areas. Comparison and validation of digital analysis output has confirmed that factorial analysis of TM band 2 digital data could be used for demarcating reef areas more accurately and the filtering and band rationing techniques are useful in mapping suspended particulate matters in coastal waters and ocean colour variations rather than mapping reef areas.
GIS based CZIS Analysis for Gulf of Mannar Biosphere
Since GIS is a powerful tool to assemble, analyse, store, utilise, retrieve, manipulate and disseminate scientific/technical data and thus aid in decision making, a GIS based Coastal Zone Information System (CZIS) for Rameswaram has been developed using PAMAP-GIS approach at the Institute for Ocean Management funded by Dept. of Ocean Development. An interim report of this study was distributed to various user departments to receive their feedback. Two user utilisation workshops have been conducted by the Institute for Ocean Management to illustrate the results obtained from this study (Ramachandran et. al., 1994). The CZIS developed using GIS for Rameswaram contains thirteen theme maps derived from remote sensing data, SOI toposheets and conventional surveys. The overlay analysis of 1968 SOI toposheet and 1990 satellite data (taking care of tidal boundaries) clearly shows the erosion and accretion sites along Rameswaram island and also the changes in area extent of other small islands in Gulf of Mannar. Subsequent ground truth work in Gulf of Mannar has confirmed that the shoreline erosion in Rameswaram leads to deposition of sediments over reef areas around Shingle, Kurusadi, Kovi and Pumurichan islands. This is an important biophysical factor to be considered for the conservation of reefs in Gulf of Mannar. Prevention of shoreline erosion in near by Pamban bridge and boat jetty areas in Rameswaram island will reduce further degradation reefs due to sediment deposition in the adjacent islands.
Conclusion
Optical Remote Sensing data and GIS are useful tools for mapping coral reefs and analyse the impact of certain biophysical factors like shoreline erosion/accretion responsible for degradation of reefs. GIS based CZIS method could be suggested for the preparation of management plans for coral reefs.
Acknowledgments
The above study was funded by the Dept. of Ocean Development and carried out under the guidance and co-ordination of Space Applications Centre (ISRO), Ahmedabad. The authors are grateful to the funding and co-ordinating agencies. Thanks due to Dr. Shailesh Nayak of SAC for his valuable input and help.
Reference
S. Ramachandran, R. Krishnamoorthy, S. Sundaramoorthy, D. Mohan and S.P. Karthikeyan (1994), Coastal Zone Information System (CZIS) Pilot Project for Rameswaram, Project Report submitted to Dept. of Ocean Development, New Delhi, 60 pp.
