This is a collection of tile-by-tile NAPP-III DOQs (1998-2001) images in MrSID format for the state of Illinois, in UTM Zones 15 and 16, NAD83. Orthophotos combine the image characteristics of aphotograph with the geometric qualities of a map. Theprimary digital orthophotoquad (DOQ) is a 1-meter groundresolution, quarter-quadrangle (3.75-minutes of latitudeby 3.75-minutes of longitude) image cast on the UniversalTransverse Mercator Projection (UTM) on the North AmericanDatum of 1983 (NAD83). The geographic extent ofthe DOQ is equivalent to a quarter-quad plus the overedge,which ranges a minimum of 50 meters to a maximum of 300 metersbeyond the extremes of the primary (NAD83) and secondary(NAD27) corner points. The overedge is included to facilitatetonal matching for mosaicking and for the placement of primaryand secondary datum corner ticks. The normal orientationof data is by lines (rows) and samples (columns). Eachline contains a series of pixels ordered from west toeast with the order of the lines from north to south.The standard, archived, USGS digital orthophoto is formattedas four ASCII header records, followed by a series of 8-bitbinary image data records. The radiometric imagebrightness values are stored as 256 gray levels rangingfrom 0 to 255. The modified, compressed ISGS digitalorthophoto is a black-and-white image in MrSID .sidformat. The target compression ratio of 15:1 maintainssufficient detail for most mapping projects.The metadata provided in the doqindex.dbftable contain a wide range of descriptiveinformation including format source information,production instrumentation and dates, and data to assistwith displaying and georeferencing the image.