The extent and height of mangroves in Kakadu National Park - an assessment based on orthorectified stereo colour aerial photography and derived digital elevation models


Internal Report 447
A Mitchell, B Donnelly, R Lucas & K Pfitzner
Supervising Scientist Division
Department of the Environment, Water, Heritage and the Arts

About the report

This report describes the processing of colour aerial photography for mapping mangroves in KNP. The project is a collaborative project between the University of New South Wales (UNSW), University of Wales and the Supervising Scientist Division (SSD). The project was aimed at obtaining baseline data on the extent and height for the majority of mangroves in KNP. Mangroves on the Wildman, West, South and East Alligator Rivers, as well as Field and Barron Islands were mapped.

eriss funded the creation of orthomosaics and digital elevation models (DEM) from aerial photography captured in 1991. One hundred and thirty-two aerial photographs (66 stereo pairs) were used to create a fine resolution “orthomosaic”. An orthomosaic is a seamless merge of imagery that is corrected to a map projection. This allows points on the imagery to be associated accurately with the ground. Orthomosaic creation is also required for change detection and GIS integration. The orthomosaics were generated at 1 m spatial resolution with an assumed positional accuracy of ± 50 m. The internal accuracy was based on the manual identification of ground control points using digitial map sheets.

From the orthomosaics, digital elevation models (DEM) were generated. A DEM displays the height of tree canopies (or land surface in bare areas). This allows the height of the mangrove canopies to be mapped which aids in mangrove species delineation. The DEMs had a height resolution of 0.5 m (± 1 m). DEMs with horizontal and vertical accuracies exceeding 0.37 m and 0.5 m (respectively) were produced where there were closed mangrove canopies or patches of denser trees. In areas where the canopy is more open or bare ground existed, there were inconsistencies in the DEM height estimation.

The attached report details the processing of the aerial photographs.