(Left) British Fire Directional Plotter (Right) German Fire Directional Plotter
These three plotters may have been in service with Machine Gun Units, Field Artillery and Foot Artillery. The three plotters work on the same principle. One half circular section represents the forward observer, the other half circular section represents the base gun/battery position. The distance between the two must be known. The distance is set on the scale by pulling the two half circular pieces apart. The distance from the observer to target is set by crossing the gun line arm with the observer arm at the range the observer sees his target. The base gun half circle is then read to indicate the gun direction (deflection) to target and the gun line arm read to find the gun to target distance. This system will seem extremely easy in comparison to US style manual gunnery with an RDP. Also, the distance between the forward observer and the battery position can be determined by the length of field phone wire stretched between them.
The British system uses degrees and the German scale pictured is in the 0-6400 scale. The British WW1 scale was also in service with the Iraqi Army after 1921. The German scale was also in service with Ottoman troops.
Ironically, both examples within my collection actually came from South West Asia.
(Below) A very similar plotter was used in the WW2 Era, again likely by Artillery and Machine Gun Units for indirect fire.
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All images, research, and text are sole property of Ralph Lovett.