Optical Calibration Targets

This is something really interesting I hadn't thought about before, about how to test, calibrate, and focus aerial cameras traveling at different speeds and altitudes. The are some optical calibration targets built as platforms across the USA which construction and geometrical composition in contrast with different fields looks also beautiful. There is a good post about at BLDGBLOG which intro says; "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." See more;

"Formally speaking, the targets could be compared to mis-painted concrete parking lots in the middle of the nowhere, using "sets of parallel and perpendicular bars duplicated at 15 or so different sizes." This "configuration is sometimes referred to as a 5:1 aspect Tri-bar Array, and follows a similar relative scale as a common resolution test chart known as the 1951 USAF Resolving Power Test Target, conforming to milspec MIL-STD-150A. This test pattern is still widely used to determine the resolving power of microscopes, telescopes, cameras, and scanners."