Corrected firing data

Corrected firing data was a term used in the U.S. Coast Artillery to refer to firing data (range and azimuth to the target) that had been corrected for various "non-standard conditions." This could include corrections to range and corrections to azimuth or deflection. Corrections could be made, variously, for the following factors:

The uncorrected firing data, to which such corrections were applied, were those derived, for instance, from using a plotting board to track the position of an observed target (e.g., a ship) and the range and azimuth to that target from the guns of a battery.

Several of the common corrections depended on meteorological data. For this reason, each Coast Artillery fort or fire command maintained its own meteorological station which transmitted an hourly meteorological message to the entire command whenever firing was anticipated. This message included a series of five- and seven-digit data blocks that reported on the temperature at a given altitude, followed by the wind speed, direction, and ballistic density of the air at each of 11 different altitude bands, running from the surface up through 30,000 ft. The higher altitude readings were needed for firings of the 12-inch coast defense mortars, which sent their shells on very high trajectories.

Once data were available on wind speed and direction, a circular slide rule-like device called a wind component indicator (see the left-most image below) was used to figure out the components of the wind that affected either the range or the deflection (direction) of the shells fired. This device yielded index numbers that were either fed to the plotting room and used to correct readings on a plotting board, were used as input to a deflection board (see below) or were telephoned to the batteries and used by gun crews to make offsets directly on the range wheels or sights of the guns themselves.

The range correction board is pictured below. This was a tabletop device (that resembled a 1940s-vintage wide-carriage mechanical adding machine, without the operating arm on the side) that was used to figure out the individual corrections that might be required for factors #1 through #7 above and to cumulate these. The result from the range correction board was fed to a slide rule-like device called a percentage corrector to obtain the corrections (if any) to be sent to the gun/s. The range correction board made use of a paper chart that was rolled onto its working surface and offered non-standard curves from which corrections could be read off. This chart had to be specific to the combination of gun, power charge, and projectile in use at the time. Values for the individual factors (#1 through #7 above) had to be obtained by plotting room personnel from battery officers or from the hourly meteorological message.

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