Description: <DIV STYLE="text-align:Left;"><DIV><DIV><P><SPAN>The primary data set consists of historical black and white aerial photographs from Illinois acquired during 1936 to 1941. Alternative years of photography are used for certain counties when a quality set from between 1936 and 1941 cannot be located. The original paper prints have been scanned and stored as TIFF format images. The online images available are MrSID format images in which the original TIFF files have been compressed at a target ratio of 12:1 with the exception of 17 counties. Alexander, Boone, DeWitt, Douglas, Edwards, Gallatin, Hardin, Jackson, JoDaviess, Massac, Menard, Perry, Pope, Randolph, Saline, Williamson and Winnebago Counties were compressed at a target ratio of 6:1. The original photographic paper prints are at a scale 1:20,000.</SPAN></P></DIV></DIV></DIV>