Charlotte Wetland Database
Summary Report
by: Leslie Morrissey and William
Sweeney, School of Natural Resources, University of Vermont
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Charlotte Wetland Database Summary Report by: Leslie Morrissey and William
Sweeney, School of Natural Resources, University of Vermont
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Introduction
Methodology
Accuracy
Wetland Classification System
Description of GIS Coverage and Data Sources
Metadata
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Introduction
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Methodology 1) Seven photo transparencies of NHAP 1:40,000 color infrared photography encompassing the Town of Charlotte were delivered to Dr. Leslie Morrissey. Accompanying and attached to the photos were seven corresponding acetate overlays on which Ms. Janice Stone (University of Massachusetts) delineated wetland boundaries. Ms. Stone's analysis was based on the Cowardin classification system (i.e. National Wetlands Inventory). The width of the original pen line was approximately equal to 20 feet in ground distance based on the scale of the air photos. 2) The seven photo transparencies and their attached overlays were digitally scanned using a backlit scanner at a resolution of 450 dpi. This process merged the overlay and the transparency into a single digital image for a total of seven digital images. 3) The seven digital images were imported into ERDAS Imagine, a commercial image processing software package. Using the advanced image rectification module, Orthomax, inherent distortions in the aerial photography were removed by registering the photo images and wetland boundaries to an existing E911 roads coverage and a USGS 1:24,000 digital elevation model. Typically, 25-30 ground control points (GCPs) were selected in support of the rectification process. For an example of a rectified CIR aerial photograph, click here (163 kb) 4) The seven rectified images were imported into ARC/INFO GIS software system. Using heads-up (screen) digitizing, the wetland boundaries were traced to create a digital format. Where boundaries differed between images, in areas of extreme topographic diversity, and in regions where the rectification process created spatial anomalies, ancillary data sources were used to best fit the boundary lines. Digitized lines were traced in the center of the pen width and in the case of linear features like riverine wetlands, linear features were transformed into polygonal shapes. 5) Final digitized polygons were checked against original acetate overlays. Attribute information was added using ArcView. Seventy-eight different wetland types were coded and 1170 individual wetlands were delineated. The digital database was created in a format compatible and consistent with the Town of Hinesburg’s wetland inventory. Prints of the final map (1:18,000) were submitted to the Town of Charlotte following initial review. Wetland coding was simplified in the paper map production to delineate major wetland types, mixed wetland classes, rivers and lakes, and upland environments. For a look at the final dataset, click here (92 kb). Locational Accuracy: The accuracy
of this wetland coverage and map is dependent on the accuracy
of the source data (NHAP aerial photos). The positional accuracy
of wetland boundaries is approximately +/- 20 feet, estimated
from the pen line width of the delineations on the CIR acetate
overlays. These mapping accuracies are many times more precise
than the National Wetland Inventory (NWI) currently used by
the state of Vermont to manage wetlands. The NWI maps are
considered to have a positional error of only +/- 500 meters
in select areas. In addition, some areas of the map
may be less accurate than others. In regions that fell on
the edge of two adjacent photos, several discrepancies were
noted in the delineations drawn on adjoining overlays. Such
wetland boundaries were digitized on the basis of the most
conservative estimate, that is, the boundary which delineated
the smallest area. The impact of this editing was minor since
it occurred only twice and encompassed less than 2 acres.
There were also several areas of extreme topography that caused
anomalies when rectified and registered. The most problematic
area existed along Lewis Creek near the town’s southern border.
These problematic boundaries should be regarded with caution
and field checked when possible. Minimum Mapping Unit: The minimum
mapping unit for this map is generally less than one acre. Classification Accuracy: Due to
variation in size and the canopy associated with some wetlands,
certain wetlands may have been omitted from the acetate overlays,
particularly those smaller than one acre. Select wetlands
were verified on-site by the Charlotte Conservation Commission,
the UVM team, and Karen Bates, a wetland ecologist from the
State Wetlands Office. For the Charlotte Wetland database,
78 wetland types were delineated based on the NWI classification
system. For map production, these 78 types were grouped into
eight separate classes based on vegetation or substrate. The
groups include: Forested Wetlands The wetland classification system
utilized here is based on the National Wetlands Inventory
classification system. More information on this system and
definitions of the individual classes can be found at Cowardin
Classification System or in: Cowardin, L. M., D.S. Gilmer, and C.W. Shaiffer. 1979. Classification of Wetlands and Deepwater Habitats of the United States. U. S. Department of the Interior, Fish and Wildlife Service, Washington, D. C. FWS/OBS-79/31. 131p. Description
of GIS Coverage and Data Source Wetlands of Charlotte, Vermont
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