How it Works…

The Irish Grid is a plane co-ordinate system based on a modified Transverse Mercator Projection. Map positions expressed in this system are based on a co-ordinate reference frame observed by two primary triangulations during the 1950’s and 60’s, and combined in one adjustment in 1975 to produce geographic positions (latitude and longitude) for the primary stations in the reference frame. This adjustment is known as the 1975 (Mapping) Adjustment. A modified Airy ellipsoid was used as the figure for the earth. The Geodetic Datum is known as the 1965 Datum, and is defined by the positions of the ten Northern Ireland primaries (as defined by the 1952 adjustment) and the positions of two primary stations in the Republic (as defined by the 1965 adjustment).

Geographic co-ordinates (F, ?) were projected onto the plane grid using standard Transverse Mercator projection formulae with Irish parameters. Secondary and tertiary triangulations, traverses and mapping control subsequently established Irish Grid (Eastings and Northings) positions relative to this co-ordinate frame, from which all mapping have been based since the mid 1970’s.

Positions on maps are expressed in two dimensions as Eastings (E) and Northings (N) relative to a false origin. Re-projection of two dimensional grid co-ordinates (E,N) back into geographic or ellipsoidal co-ordinates (F, ?) are possible by using standard formulae.

Recent measurements have confirmed the consistency of positions within the network as generally better than 25 cms (between adjacent stations) which confirms the quality of the Irish Reference system. However, modern measurement techniques use a global reference system, and these indicate that the absolute accuracy of the Irish Reference framework is everywhere better than 1 m.

Technical Information

Reference System Irish Grid
Reference Ellipsoid Airy Modified
Geodetic Datum 1965 Datum
Vertical Datum Malin Head
Map Projection Transverse Mercator
Measurement Unit International Metre

Airy Modified Ellipsoid

All geographical co-ordinates, on which the Irish Grid is based, are expressed in terms of the Airy Modified Ellipsoid, as fixed onto the 1965 Datum. This ellipsoid is based on the Airy Ellipsoid, defined in feet of bar O1, with a semi-major axis (a) of 20,923,713 feet, and eccentricity squared (e2) of 0.006 670 540 15. With metrication a conversion factor was agreed between feet of bar O1 to the International metre of 0.304 800 749 1. The Airy Ellipsoid was reduced by 35 parts per million (ppm) for the Irish reference ellipsoid, resulting in the following, standard parameters:


Semi-major axis (a) Irish Grid
Eccentricity (e2) Airy Modified

The 1965 Geodetic Datum

The Geodetic Datum of the Irish Grid is a derived one based on the positions of ten OSNI primary triangulation stations (1952 adjustment values), and the positions of three OSI primary triangulation stations fixed to their 1965 adjustment values. The 1965 adjustment was a best mean fit to the positions of the Northern Ireland Primary points as adjusted in 1952. The 1952 adjustment was based on the Principal Triangulation positions of three points in Northern Ireland: Knocklayd, Trostan and Divis.

Malin Head Vertical Datum

This is fixed as Mean Seal Level of the tide gauge at Malin Head, County Donegal. It was adopted as the national datum in 1970 from readings taken between January 1960 and December 1969. All heights on National Grid mapping since then are in International metres above this datum.

Earlier maps (e.g. County Series) used the low water mark of the spring tide on the 8 April 1837 at Poolbeg Lighthouse, Dublin. Initially fixed for County Dublin, it was adopted as the national datum approximately five years later. Heights above this datum were given in (Imperial) feet.

Malin Head datum is approximately 2.7 m above the Poolbeg Lighthouse datum.

The Transverse Mercator Map Projection

Technical Information

Ellipsoid Airy Modified
True Origin Latitude 53° 30′ 00” N
Longitude 8° 00′ 00” W
False Origin 200 kms west of true origin
250 kms south of true origin
Plane Co-ordinates of True Origin 200 000 E
250 000 N
Scale Factor on Central Meridian 1.000 035