Using image subtraction and classification to evaluate change in sub-tropical intertidal wetlands

A sub-tropical intertidal wetland in Moreton Bay, Queensland, Australia (27 degrees 52'S, 153 degrees 23'E) was physically modified in 1985 to manage a mosquito breeding problem. Monitoring before and after modification was done using field and remote sensing techniques. Images from large scale colour infrared aerial photography were analysed to evaluate impacts of the modification. Analysis was by subtracting and classifying digitized images taken before and after modification. The resulting 'difference type images' showed the types of change and their nature. These images indicated also the spatial extent of the changes. The site has become wetter as a result of increased tidal flushing, indicated by reduced spectral values. The area modified has changed less than the adjacent mangrove inlet which floods the area. Mangroves have increased in size and spatial extent, with increased spectral values generally. The changes are consistent with reported mangrove changes in other local areas and thus are unlikely to be a direct effect of runnelling.