Plotting board

A plotting board was a mechanical device used by the U.S. Coast Artillery to track the observed course of a target (typically a moving ship), project its future position, and derive the uncorrected data on azimuth (or direction) and range needed to direct the fire of the guns of a battery to hit that target. Plotting boards of this sort were first employed by the Coast Artillery around 1905, and were the primary means of calculating firing data until WW2.

Although several different types of plotting boards were used by the Coast Artillery over the years, the example described here is the Whistler-Hearn Plotting Board, Model of 1904 which was widely employed by the Coast Artillery between about 1905 and 1925. This description is primarily derived from two manuals of the period, each of which leaves certain aspects of the board's design and use unexplained. A 1940 manual also describes the Whistler-Hearn board.

The Whistler-Hearn plotting board (see Plate XXV at right, top) was a semicircular wooden table about 7.5 feet in diameter with a mechanism on top that could be configured to represent the geography of the harbor area in which it was used, including the locations of the base end stations that observed targets for the gun battery it controlled and the location of the gun/s of that battery.

The mechanism of radial arms and adjustable slides, arcs, and gears converted observations that had been telephoned in from the base end stations into firing data for the guns

The plotting board for a given gun battery was located in the plotting room for that battery (shown at right, bottom), a space often attached to an observation post or protected within a reinforced concrete bunker or casemate. It was served by a large crew (often more than a dozen soldiers) who were part of the Range Section of the battery personnel.

To locate a target, plotting board operators used two radial arms (called the primary and auxiliary arms), and "locked-in" the ends of these arms along on the notched azimuth scale that ran around the circumference of the board at the azimuths of the sightings that were telephoned in to them by the two base end stations. These locked arms and the baseline arm (along the base of the arc) then formed a triangle whose vertices were the two observation stations and the target. This located the target on the board, and its position was then marked by punching a hole in the paper placed over the plotting board. Next, another radial arm, called the gun arm, was swung over the plotted position of the target, and the resulting range and azimuth for firing were read off the gun arm and an azimuth scale on the gun arm center. These data were corrected or adjusted for some other variables, and telephoned to the gun/s.

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