A few years ago, The Center for Land Use Interpretation (CLUI) reported on the dozens of Photo calibration targets found in the USA. They are odd looking two dimensional targets with lots of lines on the of various sizes, used as part of the development of aerial photography. Mostly built in the 1950’s and 60’s as part of the US effort of the cold war.
At this point, just after the second world war, there was a huge push to get better information about the enemy. The military needed better aerial recconasance. This very problem lead to the development of the U-2 and the SR-71. As part of this, there needed to be methods of testing these planes with the big camera systems attached to them. This was before the development of digital photography, so resolution is much more difficult to test.
This is where the photo resolution markers came in. Much like an optometrist uses an eye chart, military aerial cameras used these giant markers. Defined in milspec MIL-STD-150A, they are generally 78ft x 53ft concrete or asphalt rectangles, with heavy black and white paint. The bars on it are sometimes called a tri-bar array, but they can come in all forms, such as white circles, squares, and checkered patterns.
The largest concentration of resolution targets is in the Mojave desert, around Edwards Air Force Base. This is the place most new planes were tested during this time, with the U-S, SR-71 and X-15 being just some of the planes tested there. There are a set of 15 targets over 20 miles, known as photo resolution road. There are also plenty of other resolution targets at aerial reconnaissance bases across the US, such as Travis AFB, Beaufort Marine Corps Base and Shaw Air Force Base.