Updated February 2023 with excerpts from the official coastal artillery training manual from 1932.
Often found in coastal artillery forts, this concrete pedestal would have been home to a Depression Range Finder (D.R.F.), an instrument designed in the 1880s used to calculate the distance and bearing of enemy shipping in order for the battery to engage. It represented a technology top change in coastal artillery and matched the new capability of Rifled Breech Loading (R.B.L.) guns that were soon to be deployed. To quote from the 1932 manual on Coastal Artillery Training:
The depression range finder (D.R.F.) is a portable instrument which can be set up on a concrete pedestal, specially provided for the purpose, near the guns for which it is installed. Owing to its short range (not exceeding 12,000 yards) its sphere of usefulness is strictly limited so far as counter-bombardment work is concerned.
It is a “one observer” instrument and can, therefore, be brought quickly on to the target. It is dependant oupon water-line observation and is thus easily rendered inoperative by smoke or haze; its accurancy is greatly affected by waves and by any banking up of the tide. It has to be datumed frequently on a suitable datum point to compensate for rise (or fall) of tide.
The D.R.F. should be sited so as to be out of the probable drift of the smoke and dust of action, so far as the prvailling wind will permit of this being foreseen, and should be 70 or 8- yardsto a flank in order to minimize the vibration effects in the immediate vicinity of the guns in action.
How it worked
Using trigonometry, the depression range finder used a series of known values (height above sea level, angle of depression) to calculate an unknown value (range to target). As the sea level changes with the tides, reference to tide tables was essential to ensure that the height above sea level of the D.R.F. remained accurate. As the D.R.F. station was not the actual location of the gun being fired, both distances from the D.R.F. to the gun positions were significant, as also was the magnetic bearing to the target. With this information, the guns could accurately engage enemy shipping.
Below are some of the DRF stations I have found on my adventures. They all have the same solid concrete plinth (or evidence of it) in the centre of the station, and they all have unobstructed views across the areas for which they are responsible.