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Irish Grid (IG 1975) vs Irish Transverse Mercator (ITM)

Irish Grid (IG 1975)

The Irish Grid Reference System was jointly designed by OSI and OSNI during the 1950’s to provide a common reference system for the island of Ireland.  Since equipment, observations and computation procedures can change over time, it is possible to produce different realisations of the same reference system for different epochs in time.  The last realisation in 1975 was universally adopted by the public and private sectors as the operational co-ordinate reference frame for Ireland, and has been known colloquially as the ‘Irish National Grid’ for almost 40 years. Although there have been recent attempts to change the name of these co-ordinates to ‘Irish Grid’ by OSI and OSNI, the term ‘Irish National Grid’ or just ‘National Grid’ is still in everyday use by the surveying and construction community.

The Irish Grid co-ordinate reference frame provided a coherent mapping framework for Ireland for the second half of the 20th century. However it has been superseded in 2002 by the ITM co-ordinate reference frame to provide a more rigorous and GPS compatible co-ordinate reference system for the 21st century.

Irish Transverse Mercator (ITM)

The Irish Transverse Mercator (ITM) co-ordinate reference system was jointly designed by OSI and OSNI in 2000 as part of a GPS compatible co-ordinate reference system for Ireland.

The ITM co-ordinate reference system was first realised in 2002 when ITM co-ordinates were published by the OSI geodetic website.  Consequently, the Irish Institute of Surveyors considers that ITM is an official co-ordinate reference frame, and of equal status to the Irish Grid co-ordinate reference frame since the date of publication of the ITM co-ordinates. The current use of ITM for all OSI data capture and data storage also gives credence to the face that ITM has already been adopted as an official co-ordinate reference by OSI.

Prendergast W.P: Best practice guidelines for precise surveying in Ireland: Dublin: Irish Institute of Surveyors, 2004.

The Ordnance Survey Website has publised a document to assist users of Ordnance Survey Ireland (OSi) data to migrate from Irish Grid (IG) to Irish Transverse Mercator (ITM)

The Ordnance Survey website also contains a useful converter to convert data from one co-ordinate system to another. It can be found here :

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