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We take so much for granted now I was reading the other day about 1500 giant concrete arrows strewn across America to guide mail planes pre radio/radar/IFR/GPS. They were spaced every 3-5 miles apart, painted bright yellow, measured 50-70 feet across. They also had rotating beacons mounted on 50 foot pylons. Pilots would basically follow the arrows to their destinationhttp://sometimes-interesting.com/2013/12/04/concrete-arrows-and-the-u-s-airmail-beacon-system/