1998. U.S. Geological Survey. Illinois National Aerial Photography Program (NAPP) Digital Orthophotography quarter Quadrangle (DOQ) data have been created for 102 counties in th... Illinois State Geological Survey.
1995. Abert, Curtis C. Landforms of Illinois with a shaded relief portrayal. Derived from USGS Digital Elevation Model(DEM) data. Illustrates various Illinois landforms i... Illinois State Geological Survey.