Dozens of two-dimentional aerial photo calibration targets are scattered all across the United States, according to a report by the Center for Land Use Interpretation. Constructed mostly during the 1950s and 1960s, these large outdoor charts were used as 'a platform to test, calibrate, and focus aerial cameras traveling at different speeds and altitudes.' The report points out that although some of these 'charts' are still used for some optical camera testing and calibration, they are primarily relics of the past.

GoogleEarth image of a tri-bar array at Eglin Air Force Base, Florida.

According to the report, 'the 1951 Resolution Test Chart on which [these outdoor calibration symbols are] based is more than 60 years old and was designed for film cameras, and predates high-resolution digital systems and CCDs. The arrangement and spacing of the lines is not well suited for computer analysis (it's not a continuous single row, but two or three rows of pairs), and it has other frequency and modulation issues that make determining sharpness by digital means inaccurate.'

This ground-level photograph shows one of the giant calibration targets as it appears today, at Cuddeback lake in the Mojave desert.  [photo: CLUI]

The report mentions some targets specifically, for example, 'three Tri-bar targets remaining at Cuddeback Lake are visual analog relics of the aerial viewing revolution, and they may be the only ones on public land. With dendritic cracks filling with brush, breaking through the uniformity of the 5:1 bars (each bar and space between the bars is five times as long as it is wide), the flat surfaces are peeling, crumbling and sprouting, producing dimensionality, and relief.'

(via Petapixel).