This data set contains Illinois county boundaries in line and polygon formats. The polygon attribute data include county name and number (FIPS) designations. The line attributes indicate which county lines also form the state boundary. The data were extracted from, and are redundant with, ISGS feature dataset IL_Public_Land_Survey_System. The data set is maintained as a separate entity for ease of query and display. The nominal scale is 1:62,500. As of 2003, the data are typically distributed in geographic coordinates (longitude and latitude), decimal degrees, and the North American Datum (NAD) of 1983, and this is the default spatial reference of the ArcSDE feature dataset in which the data are stored. The data were originally developed, however, in a custom Lambert Conformal Conic projection and were distributed in that coordinate system for several years. The data were digitized in the late 1960s and in 1984-85 from 7.5- and 15-minute USGS topographic quadrangles. Errors in the location of a given feature are dependent on the accuracy of the original maps and on the accuracy of digitizing. Estimates are that features have an average locational error of at least plus/minus 100 feet.