Use of stereo aerial photography for quantifying changes in the extent and height of mangroves in tropical Australia

The study investigated the use of aerial photographs, acquired in 1950 and 1991, for assessing the temporal dynamics of mangroves along the West Alligator River in Australia's Northern Territory. For both years, mangrove extent was mapped using an unsupervised classification of the digital orthomosaic and Digital Elevation Models (DEMs), or height maps, of the mangrove canopy were derived from stereo pairs. Helicopter and field observations in 1998 and 1999 respectively provided ground truth for interpreting the derived datasets. The comparison of mangrove extent revealed a substantial movement over the 41-year period, perhaps in response to hydrological changes that have resulted in a landward extension of saline conditions. Changes in the height of mangroves were observed but were difficult to quantify due to the reduced quality of the 1950 DEM. The study demonstrated the viability of using time-series of aerial photography for monitoring and understanding the long-term response of mangroves to environmental change, including hydrological variations and sea level rise.