Field Verification of Original and Updated National Wetlands Inventory Maps in three Metropolitan Areas in Illinois, USA

National Wetland Inventory (NWI) maps are the most comprehensive wetland maps in the U.S., but NWI maps are now outdated in many regions. A consortium led by Ducks Unlimited is updating the NWI for the Great Lakes/Atlantic Region. Updates are complete for several states but have not been verified extensively with field data. We used wetland maps from 129 on-site wetland delineation projects in three Illinois regions to assess accuracy of original and updated NWI maps. We used ancillary spatial data to characterize areas that were incorrectly classified and identify potential sources of error. Across the three regions, the original NWI omitted 49 % of total wetland area for wetlands greater than 0.2 ha, and 57 % of the area mapped by the NWI was non-wetland. The updated NWI omitted less wetland area (40 % omitted for wetlands larger than 0.2 ha), but only slightly improved errors of commission (55 % of mapped area was non-wetland). Polygons mapped as forested wetlands were less likely to be truly wetlands. Small (<0.06 ha) wetlands were often omitted. Errors reflect ambiguity in the definition of wetlands and technical limitations of the NWI methodology. Due to the high error rates, we recommend further refinement of regional wetland inventories.