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Illinois Historical Aerial Photography Print Scanning

Print Scanning
The primary goal of the Illinois Historical Aerial Photography (ILHAP) project is to create digital archival surrogates for the 1936 through 1941 USDA-AAA aerial photographs. Since the original silver nitrate negatives no longer exist, the photographic paper prints produced from the original film negatives must be digitized.

Quantitative testing of the image detail present on the paper emulsion of the prints was conducted to determine the optimum scanning resolution and grayscale range. It was found that little additional detail was gained by scanning at a resolution greater than above 750 dpi. An 8-bit gray-scale range was sufficient to reproduce all of the information preserved upon each photographic paper print. If the original large-format film negatives were available, the images could have been digitized using higher dpi values at an increased gray-scale range.

It is also imperative that each print in the collection be digitized only once to ensure cost effectiveness and minimize the exposure of each photographic paper print to the stresses of transport, preparation, handling, and digitization. An objective evaluation of several scanning systems was performed through a series of tests using a standard 20-step density wedge, circular target, fine gridlines, and an original 1938 aerial photography paper print. Based upon the results of this analysis, a Crosfield 646 IE drum scanning system was selected for the digitization procedures. The Crosfield scanning system was found to best maintain the original grayscale range and geometry of the original photographic paper print. Digitization procedures for the ILHAP project were conducted by Scantech Imaging and Martin Graphics of Champaign, Illinois.

Photographs were scanned with the north direction oriented up for each print. Counties flown in aneast/west direction have annotation on the left side of the print. Counties flown in a north/south direction have annotation on the top of the print. The photographs have a great deal of overlap; adjacent photographs are suitable for stereoscopic viewing.

Current Status
Print scanning was completed for all 102 Illinois counties in the Summer of 2011.

On-line file access is available for all Illinois counties. Also, Lake County has a seamless county mosaic of 1939 aerial photography available for purchase.

Image Archiving and Processing
The original scanned master files are archived in TIF format on gold standard CD-ROMs at the Illinois State Geological Survey. No additional image processing will be imposed on these files to ensure the best preservation of existing feature details.