Illinois DOQQ data is available by quarter-quadrangle in either State Plane (SP) Zones East and West, or Universal Transverse Mercator (UTM) Zones 15 and 16, NAD83. Each .zip file bundle includes a quarter-quadrangle image file, <filename>.sid, file-specific metadata,<filename>.xml, and <filename>.aux and <filename>.sdw, which are georeferencing files.