Combining Landsat ETM+ and Reef Check classifications for mapping coral reefs: a critical assessment from the southern Great Barrier Reef, Australia

Abstract

While the remote-sensing community attempts to find measures of reef "health" able to be detected and mapped using satellite image data, internationally recognized field assessments are already in place to document benthic cover, among other parameters, as an indicator of coral reef status. Reef Check is one such program, designed in 1996 as a globally applicable, rapid, field-survey protocol for coral reef health monitoring by volunteer divers (Hodgson 1999). The protocol is designed to provide a measure of reef health based on indicator species or families for fish, invertebrates, and substrates. Data collected contribute to a global-scale database on coral reef status for use in management plans (Hodgson 1999). Today, over 5,000 trained volunteer divers are led by more than 160 scientists in surveys of 1,500 reefs in 60 countries. During the survey, they record substrate type directly under a measuring tape at 0.5-m intervals on 4· 20 m consecutive transects between 3- and 10-m depth (Hodgson and Liebler 2002). The Reef Check substrate classification scheme provides information on benthic habitats such as hard coral, dead coral, soft coral, fleshy algae, rock, rubble, sand, silt sponge, and other. This is the same type of information that remote-sensing scientists are often asked to extract from image data by coral reef scientists and managers. Reef Check programs are an important source of reef-health information, yet the data collected will always be point based, and extrapolation to non-surveyed areas will be required. Remotely sensed data can provide a spatially extensive survey if methods are developed to successfully link ground-survey observations, like those provided by Reef Check, with satellite or airborne images. Thus, the objectives of our project were (1) to determine whether Reef Check substrate classes could be successfully mapped with Landsat ETM+ (Enhanced Thematic Mapper Plus) image data, and (2) to determine whether Reef Check data that are currently collected globally may be used for training and assessing the accuracy of remotely sensed image classifications. We address our objectives using a case study in the Capricorn Bunker Group, southern Great Barrier Reef, Australia.