Prior to the introduction of a new geodetic levelling of Northern Ireland in 1957, all levels were related to Poolbeg Datum. The completion and adjustment of this new levelling network prompted the introduction of a new vertical datum for Northern Ireland.
The datum chosen to relate this levelling and the later secondary and tertiary networks is referred to as Mean Sea Level (MSL) Belfast. This change was in line with accepted land surveying practice, which relates levels to mean sea level rather than the hydrographic practice of measuring height above the level of the lowest expected low tide.
To establish MSL Belfast, the Belfast Harbour Board made a series of tidal observations available to OSNI, covering the six-year period from 1st January 1951 to 31st December 1956, with readings extracted for each one-hour interval. A total of 50406 readings were accepted, and the mean value calculated as 2.0149m above Harbour Datum. This is the height of MSL above the datum of the tide gauge.
The datum of tide gauge had been adjusted so as to be in agreement with the zero mark at the 15' tide pole at the Clarendon Dock. The Harbour Board's engineer tested the relationship at approximately monthly intervals, and found the discrepancy between the two zero lines never exceeded 1". OSNI therefore accepted chart datum to be identical to tide pole zero.
The relationship between the 15' mark and the Fundamental Bench Mark at the Grove Playing Fields was established via an OS bolt.
The gauge and tidal poles have since undergone several moves within the docks. It is assumed that those responsible for the moves have ensured that the gauge and pole datums have remained on exactly the same plane.
These tidal and levelling observations enabled OSNI to establish MSL Belfast and then to relate all other benchmarks in the Northern Ireland vertical network to this datum. All levelling information published on OSNI large-scale products since the1960's relates to MSL Belfast.