English: This image shows a depression position finder (DPF) on the left and an azimuth scope on the right. Both instruments could be used to measure the azimuth of a target from the observation station; the DPF could also be used to measure the range to the target.

The DPF was normally installed with its heavy steel base bolted to an octagonal concrete pad that protruded 6 inches or so from the floor of the observation station. An azimuth scope was usually supported by a portable tripod, as shown here, although it could also be mounted to a concrete-filled steel pipe.

The "pintle center" of the DPF (the cylindrical mount-point in the center of the upper platform, with the "H"-shaped trusses to its front and rear) was taken as the point being observed from, and its latitude and longitude were precisely determined by survey techniques. This position was used in triangulating the position of the observed target. This position was often also marked by means of a brass survey marker emplaced in the roof of the fire control structure, directly above the pintle center location.

The operator of the DPF is seated on a wooden bench, using the DPF to "waterline" a target (placing the crosshairs on the ship's waterline, just under its forward mast). When the operator indicated that he had the target sighted correctly, the talker to his left (wearing the telephone set) would read the azimuth (and possibly also the distance) to the target off of dials on the DPF and repeat these data to the plotting room, where they were used in the fire control process.

The azimuth scope was often used in the process of spotting the fall of fire (shell splashes) from the battery being directed and to call out corrections to the battery's aim (over, short, left, or right). The cross-shaped wooden standard hanging on the wall behind the DPF in this photo is a guide for spotting shell splashes. Spotting was not always used, however. It was argued that adjusting fire in this manner could reduce (rather than improve) its accuracy.

This photo, taken by the U.S. Coast Artillery, has been said to be sited in a fire control station at Fort Standish on Lovells Island in Boston, MA. It has been re-cropped and adjusted for brightness and contrast by the contributor, with editing artefacts removed.