BLDGBLOG: Optical Calibration Targets
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.
http://bldgblog.blogspot.com.au/2013/02/optical-calibration-targets.html