Often found in coastal artillery forts, this concrete pedestal would have been home to a Depression Range Finder (DRF), 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 (RBL) guns that were soon to be deployed.
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 DRF remained accurate. As the DRF station was not the actual location of the gun being fired, both distance from the DRF to the gun positions was important, and also 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 consist of 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 have a responsibility.