It's the mid-1920's and you're in a plane trying to navigate your way across the vastness of the United States. GPS hasn't been invented yet. VHF Omni Directional Radio Range, shortened to VOR, hasn't been invented yet. LFR, or Low Frequency Radio Range, hasn't been invented yet. How do you hope to stay on course?
As a pilot you'd have a compass, an altimeter and maybe a map of the railway system to help you navigate and this is just what pilots did from 1918 when the U.S. Postal Service introduced the U.S. Air Mail system. But you needed one critical thing to help you navigate, one thing that wasn't available 24 hours a day. You needed daylight.
In 1921, an experimental night flight was successfully completed using the clever solution of following bonfires along the length of the route between Chicago and North Platte in Nebraska. The bonfires were lit and tended by Postal Service employees and the occasional helpful farmer.
Having proved that a regular, day and night, postal service was possible, starting in 1923 a system of beacons were built across the United States. Each beacon was a 51 foot tall tower, one every 10 miles, with a massive rotating lamp on top that could be seen up to 40 miles away. Additional lights of differing colours pointed the two directions of the route and another light flashed out the beacon's identifier, in Morse code.
All of which was essential during the hours of darkness, but to help during daylight hours, each beacon was built on top of, or alongside, a massive concrete arrow, 70 feet in length, painted bright yellow, that pointed out the direction to the next beacon.
In their heyday, almost 1,500 beacons were built between 1923 and 1933. This navigation system continued despite the invention of Low Frequency Radio Range navigation in 1929. The last beacon was supposed to be shut down as late as 1973 but some are still in use in Western Montana.
While the beacon towers themselves are mostly long gone, many of the concrete arrows still remain and can be seen clearly from the satellite imagery that we now expect to accompany today's GPS driven digital maps. The arrows may lack their trademark yellow paint as age and weathering take their toll and in a lot of cases the next beacon that they pointed to has vanished under a new development.
There's an oddly pleasing sense of continuity that a navigation aid from the pre-GPS era is still visible in the maps we now take for granted.
Air beacon courtesy of the United States Federal Aviation Administration, map imagery courtesy of Google Maps.