EM Advantages and Limitations

EM instruments accurately measure conductivity of the shallow subsurface, which is highly correlated to elevation and soil salinity. Conductivity is highly inversely correlated to lidar-derived elevation on the Mustang Island transects. As was true with elevation data, EM-derived conductivities correlate well with both mapped NWI wetland and upland habitats and coastal environments surveyed in the field. EM and lidar data achieve similar levels of detail exceeding that achieved on the NWI maps. Conductivity variations measured along each transect closely track changes in coastal environment identified during the field surveys, suggesting that EM data could be used to classify coastal environments to the same level achievable with ground-based vegetation surveys. Comparisons of mapped NWI units with conductivity data acquired along the two Mustang Island transects reveal apparent misclassifications in the NWI maps, both where mapped wetland units have conductivities that indicate an upland habitat and where mapped upland habitat has conductivities that indicate wetland environments.

There are statistical differences in conductivities measured among various wetland habitats and coastal environments. For NWI units, average conductivities increase from upland, palustrine, estuarine, and marine units according to the flooding frequency. Similar trends are observed in coastal environment classes: lowest conductivities are measured within dune and VBF environments where elevated ground is unsaturated or partly saturated with fresh water. Low and high fresh marshes have higher conductivities than dune and VBF classes. Highest conductivities are measured in salt marsh and wind-tidal flat environments.

Although average conductivities for each NWI unit and coastal environment are distinct, the ranges of conductivities measured within these units overlap to varying degrees. Upland and fresh environments are most easily distinguishable from estuarine and marine units because the conductivity strongly responds to changes in salinity. Overlap in ranges can lead to misclassification of units if the classification is based on conductivity alone. Unlike lidar, EM measurements are made at a specific location rather than over an area. Whether made from ground-based or airborne instruments, EM measurements are typically presented as profiles along a path rather than surfaces over an area.