Establishing a “spot” thematic neo-channel for the study of hard-of-access lagoon environments: Example of application on the growth areas of the new Caledonian Reefs

The analysis of SPOT channels XS1 and XS2 concerning coral reef images shows that it is possible to isolate the spectral signature of live madrepores, the main builders of the reefs that surround the lagoons of many tropical islands. The method is based on establishing a “coral index”; neo-channel, or “CI”;, whose dynamics represent the percentage of live madrepores cover for reef areas between 0 and 5 metres depths. This is applied to New Caledonia over an area of lagoon of about 1300 km2. The results include a thematic map showing the distribution of live madrepores on the numerous reefs of the area under study; its interpretation yields synoptic information on the state of health of the reefs, and their likely morphological evolution. Furthermore, the pixels representing the three levels of cover density yields quantitative data on the surface area occupied by shallow-water live coral. Critical analysis of the method and of the residts obtained in New Caledonia shows that the CI channel provides a useful tool for the long term monitoring of coral formations, which are considered as an indicator of ecological changes.