The Center for Land Use Interpretation on symbols strewn across the American landscape which make sense only to airborne machines:
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There are dozens of aerial photo calibration targets across the USA, curious land-based two-dimensional optical artifacts made mostly in the 1950s and 1960s, and many are still in use, though their history is obscure.
Most of them follow the same form established by the Air Force and NASA. The pattern painted on the targets is sets of parallel and perpendicular bars that function like an eye chart at the optometrist. For aerial photography and satellites, it provides a platform to test, calibrate, and focus aerial cameras traveling at different speeds and altitudes.
Many of these resolution test targets are found in the Mojave desert of California, one of the principal development and test areas for surveillance aircraft. The largest concentration in one place is on the grounds of Edwards Air Force Base, where calibration targets run for 20 miles.