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HISTORIC WETLANDS OF PRUDENCE ISLAND
Citation:
McGovern, D AND C Pesch. HISTORIC WETLANDS OF PRUDENCE ISLAND. Presented at Northeast ARC Users Group-16th Annual Conference, Worcester, MA, September 23-26, 2001.
Description:
Ten wetland sites around Narragansett Bay, Rhode Island have been selected for a multidisciplinary study. These wetland sites are being studied to develop indicators of "wetland health." The study includes assessing the ecological conditions of the wetlands in the past, and the cumulative effects of development on the sites. Historic maps are used to delineate the extent of the wetlands in the past and compare that to present conditions. Topographic surveys (T- charts) produced by the U.S. Coast and Geodetic Survey, predecessor to the U.S. Geologic Survey (USGS), provide a rich source of information on historic environmental features dating back to the mid 1800s. The U. S. Coast and Geodetic Survey produced these charts using state of the art technology and methods for that time. One wetland site, Prudence Island, has been selected to show the techniques needed to work with historic maps. This poster shows an historic map (T- chart, 1866) of Prudence Island with modem wetlands and coastline from the RIGIS eelgrass coverage superimposed on it. The positional accuracy of the mean high water line from the1866 map is estimated to be within a maximum error of plus or minus 10 meters. The modem coastline and wetland features have a positional accuracy of plus or minus 12.2 meters. The rectification steps used to align the historic and modem map are shown. Current bathymetry (water depth) and topography (land elevation) data were superimposed on the historic map. The bathymetry colors and hillshading were created from inverse weighted distance (IDW) interpolation of over 12,000 individual contemporary depth soundings obtained from NOAA. The land elevation colors and hillshading were created from contours visible on the historic Prudence Island 1866 T- chart. The technical challenges of working with historic maps are discussed.