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The U.S. Fish and Wildlife Service's National Wetlands Inventory (NWI) was developed to map U.S. wetlands comprehensively, following the Cowardin classification system. Wetlands are mapped by the NWI using small scale aerial photographs, but the inventory is not field checked in detail. The accuracy of this inventory has been questioned. During the summer of 1990, I field checked the portion of the NWI map of Charleston, OR Quadrangle containing the South Slough National Estuarine Research Reserve. Using the Cowardin classification, I delineated fresh and saltwater wetlands in the South Slough Reserve and identified wetland plant communities as Cowardin wetland types by field reconnaissance methods. I hypothesized that the NWI would have two kinds of errors: classification errors and delineation errors. A classification error occurs when the
Cowardin wetland type is incorrectly designated on the NWI map. A delineation error occurs when the wetland is incorrectly placed on the NWI map, omitted, or included when it was not a wetland. Of the 402.3 ha
of wetlands I field checked, 52.4 ha were classified incorrectly and 16.2 ha were delineated incorrectly. On an area-basis the NWI is about 83% accurate in this test case. Given the small scale of air photo
interpretation, I judged the NWI to be quite accurate in the South Slough area. |
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