COMBINING EM AND LIDAR TO MAP COASTAL WETLANDS: AN EXAMPLE FROM MUSTANG ISLAND, TEXAS Jeffrey G. Paine, William A. White, Rebecca C. Smyth, John R. Andrews, and James C. Gibeaut Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin Abstract We combined airborne lidar and ground-based EM induction measurements with vegetation surveys along two transects across Mustang Island, a barrier island on the Texas coast, to examine whether these methods can be used to map coastal wetlands and associated geomorphic environments. Conductivity varied inversely with elevation along both transects. Elevation and conductivity profiles cor- related reasonably well with habitat mapped in the largely imagery-based 1992 National Wetland Inventory (NWI), but they possessed greater detail and identified misclassified habitat. Detail achievable with elevation and conduc- tivity data was similar to that achieved in on-the-ground vegetation surveys. Lowest elevations and highest conduc- tivities were measured in saline environments (marine and estuarine units, forebeach, salt marsh, and wind-tidal flats). Highest elevations and lowest conductivities were measured in nonsaline environments (upland and palustrine units, dunes, vegetated-barrier flats, and fresh marsh). Elevation and conductivity data allow better discrimination among coastal wetland and geomorphic envi- ronments than can be achieved from image interpretation alone. Future work should include evaluating the effect of vegetation density on lidar-beam penetration, quantifying seasonal change in ground conductivity in fresh and saline coastal environments, examining the geographic variability of elevation and conductivity statistics, and evaluating the use of airborne EM sensors to measure ground conductivity at multiple exploration depths. Introduction We explore whether two noninvasive geophysical methods — lidar (light detection and ranging) and EM (electromagnetic induction) — can improve the accuracy and resolution of wetland mapping that has historically been based chiefly on analysis of aerial photographs and limited field checks. The importance of monitoring the status and trends of coastal wetlands has been increasingly recognized in recent decades as we have become more aware of the critical role wetlands play in the transitional aquatic-terrestrial environment and have become con- cerned about the rapid change in wetlands resulting from the historic rise in relative sea level. Using Mustang Island on the central Texas coast as an example (Figure 1), we examine the strong relationships among (1) elevation, soil and water salinity, and coastal habitat and (2) conductivity and salinity. We did this by acquiring lidar-derived elevations and EM-derived conductivities and comparing these measurements with coastal habitat and geomor- phic environment data across this sandy barrier island. We selected two representative transects across Mustang Island (Figure 1), where we acquired lidar data, surveyed vegetation type, and measured the apparent electrical conductivity of the ground. Conductivity, which is closely correlated to soil and water salinity, was measured along the transects using a ground conductivity meter. We evaluated the traditional approach to wetland mapping by comparing habitat types extracted from the most recent wetland maps with coastal environments directly observed along the transects. We evaluated the lidar and EM approach by examining the relationships along each transect among mapped wetland units, lidar-derived elevation, and measured ground conductivity and vegetation type determined during the ground surveys. We employ terms from two common classification systems to examine the relationship between elevation, conductivity, and coastal vegetation assemblages: the more technical system used by the U.S. Fish and Wildlife Service in the National Wetland Inventory (NWI) program and a geomorphic system used in our ground-based 745 Paine, J. G., White, W. A., Smyth, R. C., Andrews, J. R., and Gibeaut, J. C., 2005, Combining EM and lidar to map coastal wetlands: an example from Mustang Island, Texas, in Proceedings, Symposium on the Application of Geophysics to Engineering and Environmental Problems: Environmental and Engineering Geophysical Society, p. 745-756 (CD-ROM).