Irish Transverse Mercator
Encyclopedia
Irish Transverse Mercator (ITM), is the geographic coordinate system
for Ireland. It was implemented jointly by the Ordnance Survey 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
.
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.
.
The ITM coordinate for the Spire of Dublin
on O'Connell Street
is:
The first figure is the easting and means that the location is 715,830 metres east from the false origin (along the X axis). The second figure is the northing and puts the location 734,697 metres north of the false origin (along the Y axis)
The equivalent Irish Grid coordinate for the same location is:
The Spire of Dublin example provides a fix
for a location that is accurate to 1 metre. 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:
This ITM coordinate has three digits behind the decimal point which gives a fix for a location with millimetre 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.
With ITM there is no provision for using myriad
letters and truncated coordinates as there is with the Irish Grid.
Every coordinate must be given with at least a six digit easting and northing from the false origin.
Online Converters
Geographic coordinate system
A geographic coordinate system is a coordinate system that enables every location on the Earth to be specified by a set of numbers. The coordinates are often chosen such that one of the numbers represent vertical position, and two or three of the numbers represent horizontal position...
for Ireland. It was implemented jointly by the Ordnance Survey Ireland
Ordnance Survey Ireland
Ordnance Survey Ireland is the national mapping agency of the Republic of Ireland and, together with the Ordnance Survey of Northern Ireland , succeeded, after 1922, the Irish operations of the United Kingdom Ordnance Survey. It is part of the Public service of the Republic of Ireland...
(OSI) and the Ordnance Survey of Northern Ireland
Ordnance Survey of Northern Ireland
Ordnance Survey of Northern Ireland was the official mapping agency of Northern Ireland. The agency ceased to exist separately on 1 April 2008 when it became part of Land and Property Services Northern Ireland, an executive agency of the Northern Ireland Department of Finance and Personnel, along...
(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
Ireland
Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...
.
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
Conformal
Conformal may refer to:* Conformal map, in mathematics* Conformal geometry, in mathematics* Conformal map projection, in cartography* Conformal film on a surface* Conformal coating in electronics* Conformal hypergraph, in mathematics...
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.
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 originOrigin (mathematics)
In mathematics, the origin of a Euclidean space is a special point, usually denoted by the letter O, used as a fixed point of reference for the geometry of the surrounding space. In a Cartesian coordinate system, the origin is the point where the axes of the system intersect...
.
The ITM coordinate for the Spire of Dublin
Spire of Dublin
The Spire of Dublin, officially titled the Monument of Light is a large, stainless steel, pin-like monument in height, located on the site of the former Nelson's Pillar on O'Connell Street in Dublin, Ireland.-Details:...
on O'Connell Street
O'Connell Street
O'Connell Street is Dublin's main thoroughfare. It measures 49 m in width at its southern end, 46 m at the north, and is 500 m in length...
is:
- 715830, 734697
The first figure is the easting and means that the location is 715,830 metres east from the false origin (along the X axis). The second figure is the northing and puts the location 734,697 metres north of the false origin (along the Y axis)
The equivalent Irish Grid coordinate for the same location is:
- 315904, 234671
The Spire of Dublin example provides a fix
Position fixing
Position fixing is the branch of navigation concerned with the use of a variety of visual and electronic methods to determine the position of a ship, aircraft or person on the surface of the Earth.These techniques include:...
for a location that is accurate to 1 metre. 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 709885.081m, N 736167.699m
This ITM coordinate has three digits behind the decimal point which gives a fix for a location with millimetre 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.
With ITM there is no provision for using myriad
Myriad (unit of area)
A myriad is an area 100 km × 100 km square i.e. it is 10,000 square kilometer. 100 of these squares would be one million square kilometers....
letters and truncated coordinates as there is with the Irish Grid.
Every coordinate must be given with at least a six digit easting and northing from the false origin.
Comparison of ITM, Irish Grid and UTM
| ITM | Irish Grid | UTM | |
|---|---|---|---|
| Reference Ellipsoid: Reference ellipsoid In geodesy, a reference ellipsoid is a mathematically-defined surface that approximates the geoid, the truer figure of the Earth, or other planetary body.... |
GRS80 | Airy Modified | GRS80 |
| Central Meridian: | 8° West | 8° West | 9° West |
| Scale Scale (map) The scale of a map is defined as the ratio of a distance on the map to the corresponding distance on the ground.If the region of the map is small enough for the curvature of the Earth to be neglected, then the scale may be taken as a constant ratio over the whole map.... 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 Ordnance Datum In the British Isles, an Ordnance Datum or OD is a vertical datum used by an ordnance survey as the basis for deriving altitudes on maps. A spot height may be expressed as AOD for "above ordnance datum". Usually mean sea level is used for the datum... : |
Malin Head Malin Head Malin Head , on the Inishowen Peninsula, County Donegal, is usually given as the most northerly headland of the mainland of Ireland . In fact, the most northerly point is actually a headland named Banba's Crown on the Inishowen Peninsula about 2 km to the northeast... |
Malin Head | ? |
| EPSG Code: | 2157 | 29902 | 32629 |
External links
Online Converters
- OSI Coordinate Converter Allows conversion between Irish Grid, ITM, UTM & ETRF89.(If registration is required enter: ie@ie.ie for the email address and password for the password.)
- www.fieldenmaps.info Detailed converter: ITM,UTM, Irish Grid, War Office Irish Grid, Bonne Projection, Decimal/Deg. Min. Sec. Lat. Long. with multiple datums.
- Ordnance Survey (UK) Coordinate Converter. Click on the ITM button to toggle between Irish Grid and ITM.