Project Summary

We combine elevation data acquired using airborne lidar, conductivity measured using electromagnetic (EM) induction, and vegetation surveys to examine whether topography and ground conductivity can be used to map coastal wetland vegetation assemblages. In 2003, we used airborne lidar to acquire elevation data along two transects across Mustang Island, a modern coastal barrier on the central Texas coast. We combined the centimeter-scale elevations with ground-based conductivity measurements and vegetation surveys acquired at 20-m spacings from the gulf beach to the bay shore. It has long been known that wetland vegetation responds to both elevation and salinity; because ground conductivity is strongly influenced by soil salinity, we used EM induction measurements as a salinity proxy. Elevation and conductivity information, acquired either on the ground or from aircraft, represent a quantitative complement to traditional wetland mapping methods that rely upon aerial photographs and limited field checks.

Along both transects, conductivities were highly negatively correlated with elevations. Elevation and conductivity profiles correlated reasonably well with habitat mapped in the 1992 National Wetland Inventory (NWI), but showed greater detail than is depicted on the NWI maps and identified some areas where mapped wetland units are likely to be uplands and others where upland units are likely to be wetlands. Detail achievable with elevation and conductivity data was similar to that achieved in the ground-based vegetation surveys along each transect. Lowest elevations and highest average conductivities were measured in saline environments such as marine and estuarine NWI units, the forebeach, low and high salt marshes, and low and high wind-tidal flats. Highest elevations and lowest conductivities were measured in generally nonsaline environments such as upland and palustrine NWI units, fore- and back-island dunes, vegetated-barrier flats, and low and high fresh marshes.

Combined or individually, elevation and conductivity data allow better discrimination among coastal wetland environments than can be achieved from aerial photographic interpretation alone. Future work in the promising application of lidar and EM to rapid and accurate classification of coastal environments should include evaluating the effect of dense vegetation height on the ability to accurately determine land-surface elevation, determining the magnitude of possible seasonal change in the electrical conductivity of the ground in fresh and saline coastal environments, examining the applicability of elevation and conductivity statistics obtained for coastal environments in one geographic area to classification of similar environments in other areas, and evaluating the potential benefits of using airborne EM sensors to measure ground conductivity remotely and at multiple exploration depths simultaneously.