Analysis on coral reefs mapping using SPOT5 data at the Dongsha Atoll

Coral reefs are an sensitive-to-environment complex marine ecosystem.The ecosystem of corals is rich in biodiversity.Remote sensing offers a powerful tool for categorizing coral reefs and is the most cost-effective approach for the large-scale reef survey.The Dongsha Atoll, more than 300 km² with an average depth of 10 m, is located at the northern continental margin of the South China Sea.It has been abused by destructive fishing during recent decades.Three satellite imageries (Quickbird2, ETM+ and SPOT5) are used to evaluate the capabilities of SPOT5 imagery to provide data that are useful for categorizing the current distribution of coral reefs therein.During the data processing, unsupervised classification functions are adopted for ETM+ and SPOT5 data, while the supervised classification method is used for Quickbird2.The classes are (or not) merged into coral reef, and then will be operated by vectorization, simplification, and topological analysis.There are 1 331 coral reefs larger than 100 m² with a detection limit of 3×3 pixels at the multi-band data of Quickbird2, which is taken as the comparison baseline.The results extracted from SPOT5 and ETM images are less in number and area than those from the Quickbird2 image, whereas the results from SPOT5 data are better than those of ETM data at the silty lagoon due to its higher resolution.SPOT5 XS band 2 fails to distinguish the deep substrate inside the atoll compared with ETM data because of its poor penetration capability.Only SPOT XS band 1 cannot be used to differentiate coral reef from sand bottom.Merging the SPOT5 multi-bands data with the spatial resolution of SPOT5 pan-data and referring to ETM imagery are expected to provide an optimal satellite-based approach for mapping of coral reefs.