This is what they look like from the air:
And on the ground:
The Center for Land Use Interpretation, which supplied these photos (via BLDGBLOG, says the patterns were laid mostly in the 50s and 60s to aide with the calibration of cameras on planes such as the SR-71.
These types of calibration symbols, however, are outmoded and not terribly useful for modern digital satellite cameras.
In 2011 it was revealed that China had built a number of similar installations for the purposes of calibrating its surveillance satellites. This is what a digital surveillance camera calibration pattern looks like: