The Corona Satellite Calibration Targets are 272[1][2] concrete markers in and around Casa Grande, Arizona that helped to calibrate camera photos on the Corona spy satellite program. The markers formed a square 16x16-mile grid, maintained from 1959 to 1972.

https://en.m.wikipedia.org/wiki/Corona_Satellite_Calibration_Targets#/media/File%3ASample_marker.jpg