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Definitions and Information

What is a Trig Point?

How is a Trig Pillar Made?

What is Trigpointing?

What is a Bench Mark?

About Trigpointing Ireland

Poolbeg Datum

The first levelling of Ireland was carried out between 1839 and 1843. It was referred to a datum taken to be the level of low water of a spring tide observed at Poolbeg Lighthouse in Dublin Bay on 8th April 1837. The figures derived from this first levelling were not rigorously adjusted, and when later levelling (around 1890) incorporated marks used in the original exercise some differences in value were found. All subsequent levelling up to 1952 was brought into agreement with the previous network, which was not in itself entirely homogeneous.

Ordnance Survey
in Ireland

1824 Ordnance Survey Ireland (OSi) established. Major survey by Thomas F. Colby begun.
The History of Ordnance Survey

The Lough Foyle Base Line
The first levelling of Ireland.
Poolbeg Datum
1846Colby's survey was completed.
1867A more comprehensive survey was completed.
1921Ordnance Survey N.I. set up
1952Northern Ireland Retriangulation
1957A new vertical datum for NI.
Belfast Datum
A combined datum for OSi and OSNI was also established at Malin Head.
1959Republic of Ireland Retriangulation
1970The 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