File:Bench Mark, Belfast - geograph.org.uk - 1804479.jpg

English: Bench Mark, Belfast. A brass bolt bench mark, it is located approximately 1 metre to the south-east of the lock gates to Clarendon Dry Dock No.2 1378194. It is marked "OS BM".

The mark is 2.51 metres above MSL. See also https://www.geograph.org.uk/search.php?i=11037856 for many other examples I have found. This mark is important in the history of geodetic levelling by the Ordnance Survey Northern Ireland. Prior to a new geodetic levelling of Northern Ireland in 1957, all levels in Ireland, north and south, related to Poolbeg Datum. The completion of the 1950s levelling necessitated a new vertical datum for N.Ireland. Thus, the establishment of a new Mean Sea Level (MSL) for Belfast was created through a lenghthy series of tidal observations made at Belfast by the Harbour Board between 1951-56 and this data was shared with OSNI. A tidal gauge at Clarendon Dock was used for the observations; the relationship between the levels on the gauge and a Fundamental Bench Mark at the Grove Playing Fields in Belfast was established via this bolt on the dockside. Thus all levelling data produced on large scale maps by OSNI since the 1960s give height above MSL Belfast (smaller scale maps, such as the 1:50,000 series, refer to MSL Malin in order to ensure compatability throughout Ireland).

The information above is taken from sources produced by OSNI available online at http://www.osni.gov.uk/3.3_msl_belfast.doc and http://www.osni.gov.uk/3.2_vertical_datums_in_ireland.doc .