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'High resolution orthorectified images combine the image characteristics of an aerial photograph with the geometric qualities of a map. An orthoimage is a uniform-scale image where corrections have been made for feature displacement such as building tilt and for scale variations caused by terrain relief, sensor geometry, and camera tilt. A mathematical equation based on ground control points, sensor calibration information, and a digital elevation model is applied to each pixel to rectify the image to obtain the geometric qualities of a map. A digital orthoimage may be created from several photographs mosaicked to form the final image. The source imagery may be black-and-white, natural color, color infrared, or color near infrared (4-band) with a pixel resolution of 1-meter or finer. With orthoimagery, the resolution refers to the distance on the ground represented by each pixel. '

This six inch pixel resolution black and white aerial photography was flown April 22, 2000, at a negative scale of 1" = 833, flying at an altitude of 5000 feet. The files are provided in JPEG2000, an open format supported by most GIS and CAD software packages. Its intended usage for viewing is 1" = 100. The photography has been orthorectified to meet National Map Accuracy Standards for its capture scale. The images are georeferenced to the Illinois State Plane, Eastern Zone, using the NAD83 HARN horizontal datum. The data set is tiled for dissemination into many separate tiles. Each tile is a section in the Public Land Survey System. The first two digits are the township, the next two are the range and the final two are the section.

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