Fire control tower A fire control tower is a structure located near the U . S . coast that was used to detect and locate enemy vessels offshore , direct fire upon them from coastal batteries , or adjust the aim of guns by spotting shell splashes . These towers were part of the U . S . Coast Artillery fire control system , and were used between about 1900 and the end of WW2 . A Typical Fire Control Tower . The photo at left shows two typical fire control towers on Swallow Cave Rd . in Nahant , Massachusetts , known during WW2 as Site 131 . The tower at Site 131 - 1A ( on the right in the photo ) is the example described here . Built in 1943 - 44 , these twin towers watched over the northern approaches to Boston Harbor and directed the guns of several different Coast Artillery batteries . Towers similar to these can be found today in all coastal New England states . A fire control tower usually contained several fire control stations , known variously as observation posts ( OPs ) , base end station s , or spotting stations from which observers searched for enemy ships , fed data on target location to a plotting room , or spotted the fall of fire from their battery , so the aim of the guns could be adjusted . For example , the fire control tower at Site 131 - 1A ( diagrammed at right ) contained one OP , two base end stations , and two spotting stations . A shorthand notation was used to identify the stations . For instance , the top story of this tower was planned to contain base end station # 3 and spotting station # 3 for Battery # 15 . The overall plan document for the harbor defenses contained a list that linked the tactical numbers of all batteries to their names . That document also contained an organization chart that identified all the Command ( C ) and Group ( G ) codes , like " G3 . " These towers were arrayed in networks along the coast on either side of the artillery batteries they supported . The number and height of the towers was determined by the range of the guns involved . Many fire control towers were also part of a harbor ' s antiaircraft warning system . Spotters occupied cramped " crow ' s nests " on the top floors of the towers that enabled them to lift a trapdoor in the tower ' s roof and scan the sky for approaching aircraft . When an enemy surface craft was detected , bearings to it were measured from a pair of towers , using instruments like azimuth scopes or depression position finder s ( see photo at left ) . Since the distances along the line " between " the towers ( called a baseline ) had already been precisely measured by surveyors , the length of this baseline , plus the two bearing angles from two stations at the ends of the line ( also called base end station s ) to the target , could be used to plot the position of the target by a mathematical process called triangulation . A fire control tower was usually 5 to 10 stories tall , depending on the height of the site at which it was built and the area it had to cover . Often made of poured concrete , its lower floors were usually unoccupied and were capped by occupied observation levels . Staircases ran up to the lowest observation level , and wooden ladders were then used to climb to higher levels ( see elevation drawing above at right ) . But some fire control structures built atop coastal hills or bluffs only needed to be one - or two - story buildings , and were built of wood or brick ( see examples pictured in the Gallery below ) . Sometimes these buildings were camouflaged as private homes , and were referred to as fire control " cottages . " A plan drawing of each floor of our sample tower is shown at left . The center of octagonal concrete mounting pad on the 8th Floor ( which was meant to support a depression position finder ) was usually the surveyed point at the end of the baseline ( and thus the precise location of the base end station ) . A survey marker embedded in the tower ' s roof directly above this pad defined this point . Other observing instruments on lower floors of the tower were usually lined up directly beneath the 8th Floor mounting pad and the rooftop marker , so they shared the same latitude and longitude . The pipe stands shown on Floors 6 and 7 of the Nahant Site 131 - 1A tower probably held azimuth scopes , which were less complex telescopes that determined bearings to a target but not its range from the tower . This tower had electric lights , phones , and radio communications , and a time interval bell that was used for coordinating fire control information . Some fire control towers were also the mounting points for coast surveillance or fire control radar antennas . Although our sample tower has a simple , square appearance , some versions of these towers in New England had round or partly octagonal plans . A Network of Fire Control Structures . Each major battery of Coast Artillery guns was supported by a network of fire control structures ( towers , cottages , or buildings ) which were spread out along the nearby coast . Guns of longer range had larger numbers of fire control stations in their networks . Depending on where the target ship was located and upon other tactical conditions , one or more of these stations would be selected to control the fire from a given battery on that target . For a WW2 - era example , take Battery Murphy , the two 16 - inch guns in Nahant , MA . The map at left shows the 10 fire control stations that made up Battery Murphy ' s fire control network , which was spread out over about 40 miles of coastline running from Station 1 ( Fourth Cliff ) in the south to Station 10 ( Castle Hill ) in the north . Half of these stations were located in tall towers , and half in low - rise cottages . The length of the baselines running between each pair of stations was known very precisely . For example , Station # 1 and Station # 2 were 12 , 249 . 63 yards ( about 6 . 96 miles ) apart . These distances were plugged into the triangulation equations for the pair of stations involved in sighting on a particular target in order to compute its position . As the target ship moved along the coast , different pairs of fire control stations ( and therefore different baselines ) would come into play . Very precise measurements were also taken of the distance between the directing point of each battery ( often the pintle center of its Gun # 1 ) and each fore control station ' s observing point . These distance could also be used for target location , if one of the observations was taken from the battery itself and another from the distant station .