Introduction

This study examines whether two innovative technologies—lidar (light detection and ranging) and EM (electromagnetic induction)—can improve the accuracy of wetland mapping that has historically been based chiefly on analysis of aerial photographs. Recognition of the importance of monitoring the status and trends of coastal wetlands has increased in recent decades because of our new awareness of the critical role wetlands play in the transitional aquatic-terrestrial environment and our increasing alarm at the rapid change in wetlands resulting from the rise in relative sea level. In this pilot study on Mustang Island, we evaluate a potentially rapid and accurate wetland-mapping approach that could complement ongoing efforts in traditional aerial photographic analysis. We are exploiting (1) the known strong relationship between elevation and marsh type by comparing a lidar-derived digital elevation model (DEM) of Mustang Island with existing wetland maps and detailed vegetation transects, and (2) another known strong relationship between soil and water salinity and marsh type by collecting and comparing EM-derived conductivity data with elevation and vegetation type along the same detailed island transects.

Data used in this project include digital maps of wetland type and distribution from the 1950s, 1979, and 1992 and DEMs derived from an airborne lidar survey of Mustang Island. We compare these data sets within a geographic information system to establish the level of agreement between the wetland maps and high-resolution DEMs, which have elevation points spaced at about 1-m intervals with a vertical accuracy of about 15 cm. We selected two representative transects across Mustang Island where we surveyed vegetation type and measured the electrical conductivity of the ground. Electrical conductivity, which is closely correlated to soil and water salinity, was measured noninvasively along the transects using a ground conductivity meter. We evaluate the traditional approach to wetland mapping by comparing vegetation types extracted from the most recent wetland maps with those determined along the island transects. We evaluated the lidar and EM approach by examining the relationship along each transect between lidar-derived elevation, measured ground conductivity, and vegetation type determined during the ground surveys.