CREATING VEGETATION MAPS OF SALT MARSH VEGETATION ZONES NEAR WALLOPS ISLAND, VA, USING FIELD SURVEY DATA AND A GIS APPROACH

In May 2009, vegetation in three 50 meter X 50 meter plots, each spanning low to high marsh areas in a salt marsh near Wallops Island, VA, was intensively mapped in terms of relative abundance of all vegetation types. The goal of that larger project is to determine whether small scale changes in sea level result in shifting vegetation zones and microenvironments that can be quantified using remote sensing and GIS techniques. The purpose of the present study is to determine the optimal methodology to use when building vegetation maps in ArcGIS 9.3 from the field survey data. The vegetation maps produced may then be overlain with aerial images at different resolutions to determine what vegetation types and microenvironments may be seen. In the field survey, vegetation was observed along 50 meter transects spaced 1 meter apart and the location of any vegetation change noted to the nearest tenth of a meter, presenting some challenges in adapting the data to a GIS format. Several techniques were used to generate maps using ArcGIS. First, spatial information for the vegetation boundaries was imported from a DBF file of the raw data and digitized onto a 50 by 50 meter grid. In order to permit a fast and efficient way to create polygons representing different vegetation types, the digitized polygon grid was offset a half of a meter from the actual transect. Field data were re-classed into ten vegetation types utilizing a concatenation code for each polygon. A dissolve tool was used to condense the attribute data and merge similar polygons; a smoothing polygon algorithm (Bezier Interpolation and the PAEK method) was then applied to improve cartographic display. In addition to the composite vegetation maps, separate layers were created for each of the ten main vegetation types, making it possible to overlay the layers and see transition zones containing more than one species. The original field data ranked different plant types found in the same area by abundance, and the GIS approach offers the option for incorporating these relative abundance data into future maps. These maps can be used to assess the value of existing remotely sensed imagery in studying the characteristics of coastal vegetation in Wallops Island salt marshes.