Over at the amazing BLDGBlog, Geoff Manaugh has some photos of optical calibration targets in the California desert.
“There are dozens of aerial photo calibration targets across the USA,” the Center for Land Use Interpretation reports, “curious land-based two-dimensional optical artifacts used for the development of aerial photography and aircraft. They were made mostly in the 1950s and 1960s, though some apparently later than that, and many are still in use, though their history is obscure.”
These symbols—like I-Ching trigrams for machines—are used as “a platform to test, calibrate, and focus aerial cameras traveling at different speeds and altitudes,” CLUI explains, similar to “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.”
Further, “the largest concentration of calibration targets in one place is on the grounds of Edwards Air Force Base” in California, “in an area referred to as the photo resolution range, where 15 calibration targets run for 20 miles across the southeast side of the base in a line, so multiple targets can be photographed in one pass. There is some variation in the size and shape of the targets at Edwards, suggesting updates and modifications for specific programs. A number of the targets there also have aircraft hulks next to them, added to provide additional, realistic subjects for testing cameras.”