Developments in Salt Marsh Topography Analysis Using Airborne Infrared Photography

Abstract

Salt marshes occur throughout the extra-tropical regions of the world, along low energy shores, where a surplus of fine sediment is available, and they have been estimated as some of the most valuable ecosystems on Earth. They exhibit complex topographies primarily modelled by hydrodynamics which, in turn, determine geobiophysical processes and the pattern of occurring communities. This makes accurate salt marsh topography a prerequisite for the understanding of their function and structure. Only recently, have remote-sensing techniques become widely available to obtain high-resolution topographic data in an environment otherwise extremely arduous to access. Still, extraction of bare-earth surface remains difficult and especially problematic in areas of dense vegetation. LiDAR data, although widely in use still isn’t readily available worldwide and requires intensive post-processing and validation. A detailed digital elevation model (DEM) of the Duplin River (Georgia, Southeastern USA) was constructed with a 1 m2 resolution. The model was created by the classification and analysis of a time-series of 7 IR (infrared) aerial photographic mosaics taken at 1 h intervals from low- to high-water during a rising tide. The technique is based on the premise that flooded areas can be objectively recognized through image analysis and that the water surface is horizontal throughout the system, thereby defining a reference level at any given time. We focus on the description of the method, and results from its use in a large intertidal area. We also discuss the advantages of the method and its shortcomings when applied to vegetated intertidal areas, and propose further developments and applications.