Depression range finder

It was necessitated by the introduction of rifled artillery from the mid-19th century onwards, which had much greater ranges than the old smoothbore weapons and were consequently more difficult to aim accurately.

[6][8][7] Functionally equivalent devices were developed for the United States Army Coast Artillery Corps and its predecessors, called depression position finders or azimuth instruments depending on function (vertical base or horizontal base), adopted in 1896 and deployed widely beginning in the early 1900s as the Endicott program of modern coastal defences was built.

These calculations were built into the scales and gearing of the instrument as configured for its site, which also corrected for the curvature of the Earth and for optical parallax, so the horizontal range to the target could be read from a dial on the DPF.

It was trialled by the War Office between 1876 and June 1881, when it was formally adopted, and subsequently became standard equipment in coastal forts and batteries.

It was easy to use, highly accurate and was combined with automatic electrical updating of range and bearing dials near the guns as the position finders were manipulated.

These instruments, which contained telescopes on massive, finely geared mountings, were located in various types of fire control towers (or smaller facilities) such as base end stations, DPF bunkers, or built into concrete gun emplacements.

The American DPFs were functionally similar to the British device of the same name, but their data were usually relayed by telephone to a plotting room instead of directly to the guns.

Its effective range depended upon its height above mean low water,[i] 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.

DPFs were usually mounted on concrete columns that extended from the foundation of their stations; this minimised the effect of structure aging or minor battle damage on the instrument's position.

As longer-range guns were emplaced beginning in the 1920s, horizontal base end stations, often miles apart, became the preferred method of fire control.

[9] Coincidence range finders, self-contained short-baseline horizontal systems, began to supplement the DPF due to being quick and easy to use.

A Watkin depression position finder on a range dial in a position finding cell, Dover Castle, England
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.
An American DPF (left) and azimuth scope (right) in use
An American Warner-Swasey DPF, illustration from a 1910 manual [ 1 ]
The remains of three American base end stations with their concrete DPF mounting columns. These columns date to about 1910.
A section drawing of an American fire control building with a DPF mounting column roughly 19 ft tall.