Malin Head Datum
In 1957 the Ordnance Survey of Ireland (OSi), in co-operation with the OSNI, built a new tidal station at Malin Head in County Donegal. Following completion of the OSi geodetic levelling programme, the observations from the Republic of Ireland and from Northern Ireland were adjusted as an entire network, and related to MSL Mailn.
This adjustment was not completed until 1970, and the results allowed the offset between MSL Belfast and MSL Malin to be calculated. OSNI's choice of MSL Belfast meant the correction required to relate levels in Northern Ireland to MSL Mallin Head is small; on average MSL Malin is 0.037m above MSL Belfast.
The complete cycle of information required to establish MSL at Malin (18.6 years) has now been completed and the point has been included in the European Vertical GPS Network (EUVN).
Published OSNI Levels
All large-scale mapping undertaken by OSNI is published with metric level information referred to MSL Belfast. However, to ensure compatibility throughout the 1:50,000 and 1:250,000 map series which cover both Northern Ireland and the Republic of Ireland, the levels on all OSNI small-scale maps refer to MSL Malin.
Metric levels have been published on Ordnance Survey large-scale maps since the late 1960's, and on the 1:50 000 series and 1:250 000 map since the 1970's.