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| 1824 | Ordnance Survey Ireland (OSi) established.
Major survey by Thomas F. Colby begun.
The History of Ordnance Survey |
| 1827 to 1828 |
The Lough Foyle Base Line |
| 1839 to 1843 | The first levelling of Ireland.
Poolbeg Datum |
| 1846 | Colby's survey was completed. |
| 1867 | A more comprehensive survey was completed. |
| 1921 | Ordnance Survey N.I. set up |
| 1952 | Northern Ireland Retriangulation |
| 1957 | A new vertical datum for NI.
Belfast Datum A combined datum for OSi and OSNI was also established at Malin Head. |
| 1959 | Republic of Ireland Retriangulation |
| 1970 | The work of adjusting the observations from the
Republic of Ireland and from Northern Ireland to form one entire network related to MSL Mailn was completed.
Malin Head Datum |