Eye Charts for Airplanes
Across the USA you will find dozens of these strange markings painted on rectangular slabs of concrete and asphalt. They are known as aerial photo calibration targets, relics from the past that dot the landscape. They were made mostly in the 1950s and 60s, their form established by the US Air Force and NASA.
Aerial Photo Calibration Targets
– Typically 78 ft x 53 ft (23.7 m x 16.1m)
– Concrete or asphalt slab coated in heavy black and white paint
– Sets of parallel and perpendicular bars duplicated at approx. 15 different sizes
– Sometimes referred to as a 5:1 aspect Tri-bar Array
[Source: The Center for Land Use Interpretation]
The targets function like an eye chart at the optometrist, where the smallest group of bars that can be resolved marks the limit of the resolution for the optical instrument that is being used. For aerial photography, it provides a platform to test, calibrate, and focus aerial cameras traveling at different speeds and altitudes. The targets can also be used in the same way by satellites. [Source: CLIU]
Many of the calibration targets can be found in the Mojave Desert in California, an area where surveillance aircraft like the A12, U2 and SR71 Blackbird were developed. The largest concentration can be found on the grounds of Edwards Air Force Base located in East Kern, California. In a single 20 mile stretch across the southeast side of the base, there are 15 calibration targets in a relatively straight line. This would allow for an aircraft to calibrate and photograph multiple targets in one pass. Check out this link on Google Maps and scroll left to see the ‘run’. [Source: CLUI]
“There are an unknown number of other isolated photo calibration targets across the country, mostly inside restricted groundspace at military areas, such as at Eglin AFB, Florida; the Nevada Test Site; around Walker Field, a Navy drone airport in Maryland; and an especially exotic one at Fort Huachuca, in Arizona. Several others are painted on existing taxiways and runways, such as at Wright Patterson AFB, Ohio; Travis AFB, California; Beaufort Marine Corps Base and Shaw Air Force Base in South Carolina.” – The Center for Land Use Interpretation (CLUI)
A CLUI selection of photo calibration targets is on view in the exhibit Free Enterprise: The Art of Citizen Space Exploration in the University of California Riverside Sweeney Art Gallery from January 19 – May 18, 2013.
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