Irish Transverse Mercator - History

History

The older Irish grid reference system required GPS measurements to be “translated” (using coordinate transformations). The more precise the GPS measurements were, the more the translation process introduced inaccuracies.

While the existing UTM coordinate system partly fulfilled the requirement for direct GPS compatibility it had some drawbacks, including varying levels of distortion across the island due to the central meridian being at the west coast of Ireland.

The new system needed to satisfy various criteria: GPS compatibility, map distortion for the whole island of Ireland had to be minimal, it was to be conformal and backward compatible with existing mapping. A customized Transverse Mercator projection was chosen.

ITM and the older more established Irish Grid will (initially at least) be used in parallel. As a result ITM coordinates had to be obviously different so users would not confuse the two. This was done by shifting the ITM false origin further into the Atlantic and thereby creating substantially different coordinate numbers for any given location.

While OSI and OSNI intend to supply map information in the older Irish Grid format into the future, the Irish Institution of Surveyors has recommended that ITM be adopted as soon as possible as the preferred official coordinate system for Ireland.