This octagonal tide gauge house, located in Castray Esplanade in Battery Point and designed by Robert Huckson, was opened in December 1889, and still contains a working tide gauge where the height of tides is still measured. Twice a day, a metal float sinks into a saltwater-filled pit cut through a rock beneath the tiny building. Twice a day, the float rises with the tide.
Behind the tide-house on the right hand side of the door, one step has a square cut in it, to act as a base for a surveyor’s staff. It's the measured positioning point for the surveyor's staff. Measured at exactly 12.43 feet above sea level, the socket was the base datum point for all levels surveyed in Tasmania. This is a horizontal plane of known height to which the elevation of all other measured points are referenced.
I don't profess to really understand how this all works but the building is an interesting little building and certainly has some historical significance in the development of Hobart especially and Tasmania in general.