Champaign County Regional Planning Commission

Orthophotography

What is Orthophotography?

Orthophotography combines the image characteristics of an aerial photograph with the geometric qualities of a map. Unlike a typical aerial photograph, distortions due to relief displacement (hills, stream valleys, buildings), camera lens, and aircraft altitude have been removed so that all ground features are shown in their correct ground positions. This makes a true image map possible and permits direct measurement of distances, areas, angles, and the detailed portions of ground features that are typically omitted or generalized on traditional maps. In a digital format, orthophotography fulfills a fundamental role as a geometrically accurate base map.

Why were large X’s painted on roads all over the county?

The process of creating orthophotos requires very precise ground control. To meet this requirement, large targets (see picture) in the shape of an X are literally painted on the ground. The targets are painted precisely over know surveyed points or they are re-located with very Global Positioning System technology, (GPS). These targets show up on the aerial photos and are then used to control the accuracy of the photos by verifying that the digital image coordinates are very close to the known ground coordinates.

New Orthophotography for Champaign County

Champaign County GIS is in the process of obtaining new orthophotography of Champaign County. Similar photography for Champaign County were flown in both the spring of 2002 and the spring of 2005. New 2008 imagery is being obtained primarily because of the significant growth that has occurred in the county in the last three years that is not visible on the older photography. The new imagery will continue to serve as a base for parcel mapping, will be used to update land uses, and will function as a general reference background for multiple GIS applications.

The St. Louis office of the Sanborn Mapping Company is doing the flight and subsequent digital processing. The images will come to the Champaign County GIS already geo-referenced in the Illinois State Plane East, NAD 83 projection.

Orthophotography will be obtained at two separate scales. The entire county will be photographed at a 1:2,800 foot scale that will produce a resolution with a 2-foot ground pixel. In addition, a low flight will be done over the urbanized areas of the county. This flight will be taken at a scale of 1:660 feet that will produce a resolution with a 6-inch ground pixel.

The area of low flight (detailed 6-inch pixel) photography significantly increased with this 2005 flight compared to what was acquired in 2002. The 2002 flight included 525 low flight tiles while the 2005 flight included all of the 2002 low flight tiles plus 362 new low flight tiles.

The anticipated availability date for the spring 2008 orthophotography is December 2008. We will email the recurrent customers to let everyone know exactly when the new aerials are available for purchase as well as include notification on this page.

In 2005, Champaign County GIS acquired LiDAR (Light Detection And Ranging) data over all of the low flight tiles, and will obtained a DTM (Digital Terrain Model) and 2-foot contour data over three areas in the county.

The photos will be made available for purchase.

The Champaign County GIS Consortium is planning to put in place a schedule that would yield new countywide orthophotos every three years.