Depression position finder
![A DPF (left) and an azimuth scope (right) in use.](/Images/godic/202501/08/DPF_and_Azimuth_Scope4439.jpg")
![](/Images/godic/202501/08/Warner-Swasey_DPF4439.jpg")
![O represents the observer, at a distance OM above sea level (and the target on the sea). The range to the target is determined by taking the cotangent of the depression angle d times the height of the instrument OM, to yield the range MP. But due to the curvature of the earth, the true range is MT'. To achieve this, the DPF instrument adjusts the observer's height from OM to ON, and then correctly measures the range as NT. Alternatively, the depression angle could be adjusted to d', but this method was not used.](/Images/godic/202501/08/DPF_Angles-184439.jpg")
A depression position finder (DPF) is an observation instrument that was used in the fire control system of the U.S. Coast Artillery to locate targets in range and/or azimuth as part of the process of directing the fire of a battery of coast defense guns or mortars. These instruments, which contained telescopes on massive, finely geared mountings, were located variously in base end stations, DPF bunkers or pillboxes, or built into concrete gun emplacements. To measure range correctly, a DPF instrument had to be configured for the specific site at which it was emplaced, and had to be adjusted during the day for the level of the tide at that site. DPF instruments were used beginning in the 1890s and continuing through WW2.