Coast Artillery Fire Control
Many of the sites that were once involved in coast defense were related to do with some aspect of fire control for the Coast Artillery. This page provides a summary of the Coast Artillery fire control system, making use of links to articles on Wikipedia written by the author. Fire control sites can often be readily identified today because their locations were accurately surveyed and were marked by brass disks or other means that have survived to the present.
A key part in the fire control system was played by the base end stations that were spread out along the coast near the harbors being defended. Sometimes these base end stations took up several floors in a tall fire control tower, where they might be stacked up one on top of another. In other cases, these stations were single observation posts, located in small concrete bunkers or even in "manholes," pillboxes buried in the ground that were barely large enough for one or two people. [NOTE]
What base end stations had in common was that they sat at either end of precisely measured baselines that ran between them, and the coordinates of their observing instruments had been accurately surveyed. This meant that if the bearing angles from two of these stations to a distant target ship were measured (since the length of the line between them was precisely known), then the mathematical process called triangulation could be used to calculate the coordinates of the target ship (in terms of latitude and longitude or a point on the X-Y map grid that had been established for each harbor). [NOTE]
Prior to WW2, and in the early years of that war, observations from the base end stations were telephoned in to a plotting room, where a plotting board was used to figure out the firing data (range and azimuth) for a battery of guns firing on a specific target. A similar procedure was used to fire mortars or the submarine mines that protected the harbors. The plotting board was a mechanical device (with lockable brass arms, pivots, dials and indicators) which represented an analog of the territory near a specified Coast Artillery battery. Manipulating this board produced firing coordinates that could be telephoned to the battery commander and used to aim the guns. The target was tracked over time, and corrected firing data could be produced, accounting for factors like wind speed and direction or spotting the actual fall of fire (over, short, left, or right).
Actually, the ploting board produced firing data that referred to one specified geographic location near the battery, known as the directing point of the guns. This point was sometimes marked by a survey disk or mark and was sometimes the "pintle center" of the No. 1 gun in the battery (the one on the right, as it faced the sea). Then, depending on the separation among the guns, firing coordinates had to be further adjusted to apply to each individual gun.
Later in WW2, firing coordinates could be produced by feeding observations from the base end stations (or from fire control rader stations) directly into gun data computers, electro-mechanical devices that replaced the plotting boards.