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Description: This dataset started as a copy of the 2004 Illinois county boundaries from the Illinois State Geological Survey (ISGS). In 2005, ISWS edited the county boundaries to better match the boundaries as seen on georeferenced United States Geological Survey (USGS) 7.5-Minute Series Topographic Maps. Aerial photography was also used to assist in this task. In 2023, all counties along the state boundary with Indiana were edited to match with county boundaries provided by the Indiana DNR. Also in 2023, portions of the boundaries of Lake and McHenry Counties were edited to match with their neighboring Wisconsin counties as seen in FEMA's National Flood Hazard Layer (NFHL). In 2024, Madison, St. Clair, and Monroe counties along the state boundary with Missouri were edited to match with Missouri county boundaries in the effective NFHL.
Copyright Text: Illinois State Water Survey, a division of the Prairie Research Institute at the University of Illinois at Urbana-Champaign