Depression position finder 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 station s , 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 . How It Worked . A depression position finder ( DPF ) measured the range to a distant target ( such as a ship ) by solving a right triangle in which the short side was the height the instrument above mean low water , one angle was the constant right angle between the short side and the plane of the ocean , and the second angle was the depression angle from the horizontal of the instrument as it sighted down from a fire control tower or a base end station at the target . These calculations were " built into " the scales and gearing of the instrument , which also corrected for the curvature of the earth and for optical parallax , so the horizontal range to the target could be read off of a dial on the DPF . One soldier looked through the telescope ( a 12 or 20 - power instrument ) and " waterlined " the target ship , putting a vertical cross hair on the ship ' s forward stack and a horizontal crosshair on the ship ' s waterline . By turning a geared crank , he attempted to hold the ship in his sights as it passed through his field of view . A second soldier read azimuth and range data from the instrument at designated intervals . These intervals ( usually set at 20 or 30 seconds ) were indicated by the ringing of a time - interval bell or buzzer ( shown in the image at right on the ceiling over the instrument ) . This information was then called in by telephone to a fire control center or plotting room for the gun battery that had been selected to fire upon the designated target . The DPF instrument was meant to be used to locate targets at ranges of between 1 , 500 and 12 , 000 yards . Its effective range depended upon its height above mean low water , the viewing conditions ( lighting , weather , fog or smoke ) and upon the skill of its operators in holding a " sight " on a target . From about 1900 to 1925 , DPF instruments were often mounted for stability on massive , octagonal concrete columns perhaps two feet across and buried deeply in the ground . A wooden or lath and plaster fire control tower or base end station was then built up around , but not connected to , the column . The DPF could be used as part of a " vertical " base system of triangulation to compute the range to the target . It could also be used as part of a " horizontal " base system , in which it served as one of two base end stations , both of which measured an observing angle to the target , with the range and azimuth of the target being calculated from their joint observations . During the early part of the 20th Century , DPF instruments were often installed in the Battery Commanders ' stations for coast artillery batteries and were used by the Battery Commander or a member of his staff to yield firing data ( range and azimuth , or direction ) for the guns . This system was generally much less accurate than the horizontal base system ( based on observations from two widely separated base end stations ) , and became a back - up system that was used only in emergencies ( such as damage to other observation stations ) .