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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

What is a Trigpoint?

It is usually a hand-cast concrete pillar, 4 feet high and 2 feet square at the base, tapering towards a flat top. It is part of a network of surveying stations that were used by Ordance Survey for mapping. Each pillar is in view of several others so that their position may be triangulated.
Hence the formal term is triagulation pillar, but this is usually shortened to trig pillar or trig point. On this site this is abreviated even further to TP.

TPs usually have a 'spider' or 'top plate' used to fix a theodolite or other ordnance surveying device to. Many have a Flush Bracket fixed to the side that has an identifying number on it. There are different designs, as you will discover as you read condition reports and see the photographs that these contain.

Below left is a spider and at the right, one type of flush bracket. It is attached to the Triangulation Pillar at Carncormack located at D 1691 1433.

       

Ordnance Survey
in Ireland

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
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