Irish Transverse Mercator
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Irish Transverse Mercator (ITM), is the new geographic coordinate system for Ireland. It was implemented jointly by the Ordnance Survey of Ireland (OSI) and the Ordnance Survey of Northern Ireland (OSNI) in 2001. The name is derived from the Transverse Mercator projection it uses and the fact that it is optimized for the island of Ireland.
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[edit] The need for a new coordinate system
With the huge influence of GPS and its increasing importance in industry and leisure there was a need to create an Irish coordinate system which was directly compatible with GPS.
The older Irish Grid 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. So it was decided to create the new bespoke ITM system.
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 as it fulfilled these criteria.
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.
[edit] Examples
An ITM coordinate is generally given as a pair of two six digit numbers (excluding any digits behind a decimal point which may be used in very precise surveying). The first number is always the easting and the second is the northing. The easting and northing are in meters from the false origin.
The ITM coordinate for the car park at Newgrange passage grave in county Meath is:
- 700619
- 772598
The first figure is the easting and means that the location is 700,619 meters east from the false origin (along the X axis). The second figure is the northing and puts the location 772,598 meters north of the false origin (along the Y axis)
The equivalent Irish Grid coordinates for the same location (derived from the OSI converter and rounded to the nearest metre) are:
- 300690
- 272580
The Newgrange example provides a fix for a location that is accurate to 1 meter. With ITM it is possible to give a more accurate coordinate for a given location by using a decimal point after the initial six figure easting and northing.
The ITM coordinate for the passive GPS station at the OSI office is:
- E 709885.081 m
- N 736167.699 m
This ITM coordinate has three digits behind the decimal point which gives a fix for a location with millimeter accuracy. Also notice how the easting in this example is indicated with an “E” and likewise an “N” for the northing. The fact that the coordinate is in meters is indicated by the lowercase m.
[edit] Comparison of ITM, Irish Grid and UTM
| ITM | Irish Grid | UTM | |
|---|---|---|---|
| Reference Ellipsoid: | GRS80 | Airy Modified | GRS80 |
| Central Meridian: | 8° West | 8° West | 9° West |
| Scale on Central Meridian: | 0.999 820 | 1.000 035 | 0.999 600 |
| True Origin: | 53° 30’ North; 8° West | 53° 30’ North; 8° West | 0° 00’ North; 9° West |
| False Origin: | 600,000m West; 750,000m South | 200,000m West; 250,000m South | 500,000m West; 0m South |
| Vertical Datum: | Malin Head | Malin Head | ? |
[edit] References
- Irelands Surveying Infrastructure for the 21st Century by William Patrick Prendergast.
- A New Coordinate System for Ireland-OSI
- New Map Projections for Ireland-OSI
- Coordinate Positioning Strategy-OSI
- Map Projections
- OSi Passive GPS station coordinates Registration required, but a made-up email address will suffice (Or enter: ie@ie.ie for the email address and password for the password.)
- OSi Coordinate Converter Allows conversion between Irish Grid, ITM, UTM & ETRF89. Registration required, as above.
