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Assessment of Geographically Isolated Wetlands in Eastern NC and SC

Breda Munoz 1, Virginia M. Lesser 2, John Dorney 3 and Frank Obusek 4

1 Research Statistician. RTI International, Research Triangle Park, NC
2 Associate Professor, SRC Director, Oregon State University, Corvallis, OR
3 Supervisor, Wetlands Program Development Unit, NC Division of Water Quality, Department of Environment and Natural Resources, Raleigh, NC
4 Geospatial Imaging, Center for Geographic Information and Analysis, NC Department of Environment & Natural Resources, Swannanoa, NC

Wetlands provide significant environmental benefits such as assimilation of pollutants, flood water storage, water recharge and fish and wildlife habitat. Geographically isolated wetlands (GIW) can provide the same benefits as wetlands in general, and are particularly vulnerable to losses from urbanization and agriculture because they are geographically isolated and have varying amounts of regulatory protection. Currently, there is not a dependable and cost-effective method to generate an accurate GIW map without sending a field scientist to perform surveys or requiring image technicians to perform digitalization of aerial photography. By using statistically valid estimates of accuracy rates one can evaluate the quality of the information contained in GIW maps. Accuracy rates are used to describe the misclassification errors of the maps. A probability sampling survey methodology that balances statistical considerations, expert opinion and operational considerations is proposed for assessing the accuracy of GIW maps. The proposed sampling design is based on a stratified multi-stage sampling design that addresses sampling size requirements for the different strata and types of GIWs. The sampling design also recognizes the need for spatial coverage while minimizing operational efforts. Expressions for design-based accuracy estimates and an estimate of the number of GIWs, as well as their corresponding variances are also provided.

A simulation exercise is used to illustrate the proposed sampling methodology. A GIW map for two eastern North Carolina counties (Robeson and Columbus), created using historical data, was used as the sampling frame. The GIW map was created from a combination of satellite imagery, classification tools to process the imagery and auxiliary information. The sampling methodology was used to randomly select sites from this GIW map. An updated GIW map for the same counties showing exact locations of GIW was used to provide “ground-truth” observations from wetland delineations approved by the US Army Corps of Engineers. Survey based accuracy estimates were calculated by comparing site classification differences, obtained by using both the original and updated GIW maps.

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