Directing point

The directing point (DP) was a term used in the United States Army Coast Artillery Corps to identify a precisely surveyed point that was used as the point of reference for preparing the firing data used to aim the guns of a given Coast Artillery battery.

In a four-gun battery, the DP was often located midway between the two central guns.

[3] Sometimes the DP was marked by a survey disk or a copper bolt, set by the U.S. Army Engineers (USE) or by the United States Coast and Geodetic Survey (USC&GS), often in the concrete blast slope or apron or in the ground in front of the parapet of the battery.

The photo at left below illustrates a DP like this, between the two guns of Battery Stevenson at Fort Warren in Boston Harbor.

If the guns were relatively close to each other, then it might be enough to point them all at the same azimuth, to fire a salvo in parallel with each other.

A cutaway drawing of a battery's fire control system.
A cut-away drawing of a Coast Artillery battery, showing its two base and stations (upper left), the plotting room (with plotting board), and the directing point (DP) (between the two guns, at right). The directing point, on the surface of the blast apron, is indicated by the red dot. Each gun's displacement (the distance from the pintle center of the gun to the DP) is also indicated.
A photo of a survey mark.
A copper bolt set in concrete marks the DP for the mortars of Battery Whitman at Fort Andrews in Boston Harbor.
The directing point for Battery Stevenson at Fort Warren, Boston Harbor, set in the grass strip between its two guns.