Irish Transverse Mercator (ITM) is the
geographic coordinate system
The geographic coordinate system (GCS) is a spherical or ellipsoidal coordinate system for measuring and communicating positions directly on the Earth as latitude and longitude. It is the simplest, oldest and most widely used of the various ...
for Ireland. It was implemented jointly by the
Ordnance Survey Ireland
Ordnance Survey Ireland (OSI; ga, Suirbhéireacht Ordanáis Éireann) is the national mapping agency of Ireland. It was established on 4 March 2002 as a body corporate. It is the successor to the former Ordnance Survey of Ireland. It and the ...
(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 optimised for the island of Ireland.
History
The older
Irish grid reference system
The Irish grid reference system is a system of geographic grid references used for paper mapping in Ireland (both Northern Ireland and the Republic of Ireland). The Irish grid partially overlaps the British grid, and uses a similar co-ordinate sy ...
required GPS measurements to be "translated" (using
co-ordinate transformations). The more precise the GPS measurements were, the more the translation process introduced inaccuracies.
While the existing
UTM co-ordinate 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 customised 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 co-ordinate 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 co-ordinate system for Ireland.
Examples
An ITM co-ordinate 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
A projected coordinate system, also known as a projected coordinate reference system, a planar coordinate system, or grid reference system, is a type of spatial reference system that represents locations on the Earth using cartesian coordina ...
and the second is the
northing
A projected coordinate system, also known as a projected coordinate reference system, a planar coordinate system, or grid reference system, is a type of spatial reference system that represents locations on the Earth using cartesian coordina ...
. The easting and northing are in metres from the false
origin
Origin(s) or The Origin may refer to:
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* Origin (comics), ''Origin'' (comics), a Wolverine comic book mini-series published by Marvel Comics in 2002
* The Origin (Buffy comic), ''The Origin'' (Bu ...
.
The ITM co-ordinate for the
Spire of Dublin
The Spire of Dublin, alternatively titled the Monument of Light ( ga, An Túr Solais), is a large, stainless steel, pin-like monument in height, located on the site of the former Nelson's Pillar (and prior to that a statue of William Blakeney) ...
on
O'Connell Street
O'Connell Street () is a street in the centre of Dublin, Republic of Ireland, Ireland, running north from the River Liffey. It connects the O'Connell Bridge to the south with Parnell Street to the north and is roughly split into two sections ...
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
A Cartesian coordinate system (, ) in a plane is a coordinate system that specifies each point uniquely by a pair of numerical coordinates, which are the signed distances to the point from two fixed perpendicular oriented lines, measured in ...
)
The equivalent Irish Grid co-ordinate for the same location is:
:315904, 234671
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 co-ordinate for a given location by using a decimal point after the initial six figure easting and northing.
The ITM co-ordinate for the passive GPS station at the OSi office is:
:E 709885.081m, N 736167.699m
This ITM co-ordinate 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 co-ordinate is in metres is indicated by the lowercase m.
With ITM there is no provision for using
myriad
A myriad (from Ancient Greek grc, μυριάς, translit=myrias, label=none) is technically the number 10,000 (ten thousand); in that sense, the term is used in English almost exclusively for literal translations from Greek, Latin or Sinospher ...
letters and truncated coordinates as there is with the
Irish Grid.
Every co-ordinate must be given with at least a six-digit easting and northing from the false origin.
Comparison of ITM, Irish Grid and UTM
See also
*
ETRS89
*
GRS80
The Geodetic Reference System 1980 (GRS 80) is a geodetic reference system consisting of a global reference ellipsoid and a normal gravity model.
Background
Geodesy is the scientific discipline that deals with the measurement and representation ...
*
Spatial reference system
References
Irelands Surveying Infrastructure for the 21st Century{dead link, date=January 2018 , bot=InternetArchiveBot , fix-attempted=yes by William Patrick Prendergast.
A New Coordinate System for IrelandOSi
New Map Projections for IrelandOSi
Coordinate Positioning StrategyOSi
Map ProjectionsOSi Passive GPS station coordinates(If registration is required enter: ''ie@ie.ie'' for the email address and ''password'' for the password.)
External links
OSiOSNIIrish Institution of SurveyorsOSi: Migrating to ITM
Online converters
OSi Coordinate ConverterAllows conversion between Irish Grid, ITM, UTM & ETRF89. (If registration is required enter: ''ie@ie.ie'' for the email address and ''password'' for the password.)
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.
Geographic coordinate systems
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