"Beacon": In Surveying and Benchmark hunting, there might be several meaning for the term "beacon". Here are several. There might be more:
Airway Beacon: When early aircarft began to fly longer distances than down the road to the next town, there were no aids to help pilots find their way around. Pilots essentially flew by looking out of the window at the ground, and sometimes used whatever maps they could find (such as early automobile road maps). This was OK for daytime, but airmail pilots flew at night also. Some pilots (and their friends on the ground!) began using bonfires, which became the first artificial "beacons" for night flying. By about 1923, ideas were tried like lighted airport boundaries, spot-lit windsocks and signs, etc. To support airmail flying, the US Post Office worked to complete a transcontinental airway of beacons on towers spaced 15 to 25 miles apart all the way across the country, each with enough brightness to be seen for 40 miles in clear weather. Each tower had site numbers painted on it for daytime identification. At night, the beacons flashed in a certain sequence so that pilots could match their location to the printed guide that they carried. Typically, there was a rotating beacon (like a lighthouse, but smaller) on the tower, one fixed tower light pointing to the next tower and one to the previous tower, forming an aerial roadway. After World War II, these [http://www.navfltsm.addr.com/howitbegan.htm Airway Beacons] were eventually replaced by various radio (electronic) navigation systems, such as [http://www.navfltsm.addr.com/ndb-nav-history.htm Four-Course Radio Ranges], [http://www.navfltsm.addr.com/ndb-nav-adf-1.htm ADF / NDB Radios], [:VORTAC:VORTAC ]stations, and eventually ["GPS"]!
Obviously, Airway Beacon sites had to be accurately located, and so their locations were surveyed, and were incorporated into the US Geodetic survey marker. Although most Airway Beacons have been torn down (or allowed to fall down!), there are a few still standing, and there are efforts to save the remaining ones. The NGS (and Geocaching) benchmark database still contains a number of Airway Beacons (or their old towers), and they are fun to find! An example of one with what seems to be an original tower (although the beacon light is gone) is [http://www.geocaching.com/mark/details.aspx?PID=DX3797 DX3797] in Riverside County, CA.
Airport Rotating Beacon: Although these beacons "evolved" as part of the Airway Beacon system described above, they still are a requirement at every (public) airport in the US! The [http://www.airwaysmuseum.com/Rotating%20Beacon%20BUD.htm rotating beacon] (like on top of a lighthhouse, but smaller) is required to be elevated for visibility reasons, so it is usually on top of a control tower, a hanger or other airport building, or sometimes on a stand-alone metal tower. The high intensity light is clear (white) in one direction, and green in the other direction. This results in the pilot seeing alternate flashes of white and green. The light beams are tilted up somewhat, so from ground level they don't look very bright, but are usually visible even from outside the airport. An interesting factoid: For military airfields, the white side of the beacon is "split" by a divider plate, so that when the pilot sees it, the white light is seen as a "double flash"; this allows easier identification of military versus civilian airports. Nice to know!
Like the Airway Beacons above, Airport Rotating Beacons had to be surveyed for accurate locations, and many (maybe all?) are in the NGS / Gecaching Benchmark database. Since the early survyors often worked at night using lights as targets, rotating beacons made very handy marks and probably had other survey uses as well. Usually, the center of the beacon light beam (not the top of the structure) is the actual ["station"] mark. They are normally [:intersection station:intersection stations], and there is no metal disc "benchmark". An example of one is [http://www.geocaching.com/mark/details.aspx?PID=DX4794 DX4794.]