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A plotting room (see photo at top right) was used by the U.S. Army Coast Artillery Corps and the Coastal Forces of the Royal Canadian Navy to house a team of soldiers who were engaged in controlling fire for the guns of a coastal artillery battery.
Plotting rooms were (and continue to be) used aboard ships to control fire for naval guns, but this use is not described here.
A plotting room was connected by telephone lines (and sometimes by radio) to base end stations (at left, top) that observed the locations of enemy ships and sent data to plotting room soldiers who used equipment like a plotting board to calculate where the guns should be pointed and when they should be fired.
Telephone lines also ran from the plotting room to the guns and were used to relay firing data. Other devices, like range correction boards or deflection boards, were used in the plotting room to calculate corrected firing data or to adjust range and azimuth after spotters in remote observing stations had seen where prior shots had fallen.
Plotting rooms were sometimes made of concrete and buried below ground (for protection) or were located in the reinforced concrete casemates of Coast Artillery batteries. The casemated version shown at left was built into the side of a sloping embankment near its mortar battery.
Plotting rooms were also located in free-standing structures, either low towers (like two-story tower from 1904 shown in a photo and in plan view at right) or one- or two-story wood and plaster buildings which might house facilities for several batteries right next to each other in barracks-like structures. These multiple-battery installations might also have sleeping quarters and latrine facilities built nearby.
Sometimes plotting rooms were located hundreds of yards from the batteries they controlled. They often sat on top of nearby hills or ridge lines.