Australian Army Museum of Western Australia > Records

World War 2, Western Australia, Rottnest Island, Coast Artillery Instrumentation, Depression Range Finder

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Coast Artillery Fire Control - Target and Projectile Coincidence

Overview

The depression range finder (DRF) is a precision fire control device used to observe the target's range and bearing in order to calculate firing solutions. It was the main component of a vertical base range finding system It could provide both range and bearing information on a targets. To ensure accuracy, the range finders were accurately surveyed and placed on fixed mountings. Instructions on the mounting pillars contained details of bearings to prominent reference points so that orientation could be checked. Periodic adjustments were made to take into account changes in vertical height due to tides.

Historical information

This form of instrument 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. The DRF was invented by Captain HSS Watkin of the Royal Artillery in the 1870s and was adopted in 1881.

Details

Registration number

cwa-org-32-54-760

Inscriptions and markings

A Depression Range Finder measures the range to a distant target (such as a ship) by solving a right triangle in which the short side was the height of 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, hence the "vertical base" system of range finding. 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 instrument.

Links to additional resources

Virtual Visit - Depression Range Finder

Australian Army Museum of Western Australia

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