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 and1991, for assessing the temporal dynamics of mangroves along the WestAlligator River in Australia's Northern Territory. For both years,mangrove extent was mapped using an unsupervised classification of thedigital orthomosaic and Digital Elevation Models (DEMs), or height maps,of the mangrove canopy were derived from stereo pairs. Helicopter andfield observations in 1998 and 1999 respectively provided ground truthfor interpreting the derived datasets. The comparison of mangrove extentrevealed a substantial movement over the 41-year period, perhaps inresponse to hydrological changes that have resulted in a landward extensionof saline conditions. Changes in the height of mangroves were observedbut were difficult to quantify due to the reduced quality of the 1950 DEM. The study demonstrated the viability of using time-series of aerialphotography for monitoring and understanding the long-term response ofmangroves to environmental change, including hydrological variations andsea level rise.