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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

The Retriangulation of Ireland.

Ordnance Survey of Ireland began their retriangulation in 1959. Many of the points observed in the principal triangulation of the nineteenth century were incorporated in the new scheme. Triangulation stations, in the form of the familiar concrete pillars seen on many hill tops, were erected.The Irish Grid co-ordinated values of triangulation points are available to the public and are used in many engineering and other special surveys.

Some equipment of the period.

One of the problems was transporting this heavy equipment up to some of the highest mountain tops in Ireland. Donkey power was often used for the job.

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