An interesting bit on BLDGBLOG about optical calibration targets. These cold war relics were used to test the photographic equipment on spy aircraft, and consist of sets of lines of different sizes and spacings painted on the ground, with the resolution of the camera being measured based on the finest set it can resolve.
Although I am truly fascinated by what sorts of optical landmarks might yet be developed for field-testing the optical capabilities of drones, as if the world might soon be peppered with opthalmic infrastructure for self-training autonomous machines, it is also quite intriguing to realize that these calibration targets are, in effect, ruins, obsolete sensory hold-overs from an earlier age of film-based cameras and less-powerful lenses. Calibrating nothing, they are now just curious emblems of a previous generation of surveillance technology, robot-readable hieroglyphs whose machines have all moved on.
Optical Calibration Targets [BLDGBLOG]