The adoption of regional or local reference ellipsoids results in different positions for the same point along common boundaries between two different regions. The development of a global reference system was largely driven by international military requirements, and resulted in the World Geodetic System, WGS84. WGS84 is a geocentric reference ellipsoid and a geodetic datum, in that it defines the centre of mass of the earth as its origin, and the direction of the earth’s axis as the minor axis of the reference ellipsoid. WGS84 is a further refinement of the GRS80. Although WGS84 was originally defined to a precision of 1m in any axis, more precise reference systems have been defined internationally (such as ITRF) and regionally (such as ETRF) using the GRS80 ellipsoid and high precision satellite observations at global and regional geodetic observation facilities.
|Reference Ellipsoid||Defining Parameters|
|WGS84||a = 6 378 137.000
e2 = 0.006 694 379 9
|GRS80||a = 6 378 137.000
b = 6 356 752.314 1
e2 = 0.006 694 380 022 90
The high precision geodetic global reference frame is known as the International Terrestrial Reference Frame (ITRF), with positions expressed as three dimensional earth centred Cartesian co-ordinates. Geographic co-ordinates (latitudes and longitudes) are based on the GRS80 ellipsoid as a best fitting global figure for the earth. Within Europe a network of permanently recording geodetic facilities have enabled a precise subset of the ITRF to be established, known as the European Terrestrial Reference Framework (ETRF). Due to the precision of modern measurements, movements of stations have been detected between sets of observations taken at different times and may result in inconsistency of positions observed at different times over a few years duration. Thus different co-ordinate systems are established, within the defining reference frame, linked to the epoch of the observation, and transformations have been defined between them, which move positions from those determined at the time of observations to a common system at a defined epoch. The adopted reference system for Europe is known as the European Terrestrial Reference System, 1989 (ETRS89).
Co-ordinates in ETRS89 are expressed as either three dimensional (X, Y, Z) Cartesian co-ordinates or as three dimensional ellipsoidal co-ordinates (F, ? and H, Ellipsoidal height), based on the GRS80 ellipsoid.
During the late 1980’s and early 1990’s the increased use of GPS for mapping and scientific work highlighted the need for a reference network compatible with the new technology and essential to relate the mapping framework to global and continental reference systems. An early attempt to do this based on three Primary stations was not a complete success due to faulty equipment and an immature satellite constellation. Therefore in April 1995 a new geodetic and survey control network (IRENET) was observed using the Global Positioning System (GPS). This network consists of 12 new ‘zero-order’ control stations (8 in the Republic, 3 in Northern Ireland and one on the Isle of Man) connected to some of the defining International Terrestrial Reference Frame (ITRF) stations in Europe. The resulting adjustment was accepted as an official extension to the European Terrestrial Reference System (ETRS) by sub-commission X (EUREF) of the International Association of Geodesy (IAG) in Ankara, Turkey, 1996.
These zero order stations were used to control a densification of the network to a further 173 stations throughout Ireland between May and December of the same year. Co-ordinates have been computed in terms of ETRS89 and Irish Grid (1975 Mapping Adjustment), and these stations will form the basis for all future scientific and mapping control work.
ITM is a newly derived GPS compatible mapping projection that is associated with the ETRS89 and the GRS80 ellipsoid. The true origin and central meridian defined in the Irish Grid is maintained, thus distributing the distortions due to the projection evenly.
Consideration was given to the introduction of a scale factor of unity on the central meridian. However, using a scale of 0.99982 results in two standard parallels, and the magnitude and effects of scale change are minimised.
The position of the false origin is moved to a point 600,000m west and 750,000m south of the true origin. This results in grid co-ordinates that are significantly different from IG, but does not introduce additional distortion or complexity. The magnitude of the shift ensures that IG co-ordinates plotted on the ITM projection do not fall on Ireland or Northern Ireland, and vice versa.
|Central Meridian||8° West|
|Scale on CM||0.999 820|
|53° 30′ North
8° 00’ West
|False Origin (metres)||600 000 W
750 000 S
Universal Transverse Mercator (UTM) is an internationally recognised and widely available standard projection in mapping and GIS software. It was adopted in 1947 by the US Army, and used for military maps throughout the world. It divides the earth into sixty zones, between latitudes 84° North and 80° South. Each zone is 6° wide, with a scale factor of 0.9996 applied on the central meridian.
Ireland is situated in UTM Zone 29, which has a central meridian 9° West of Greenwich, resulting in a small part of Counties Antrim and Down in the east of Northern Ireland extending outside the nominal zone width boundary of 6° West of Greenwich. However, the zone width may be altered to meet local circumstances and since the UTM grid has a standard zone overlap of 40 km on either side of a zone boundary, all of Ireland can be contained within Zone 29.
Since the central meridian lies along the West Coast of Ireland, mapping distortions are not distributed evenly. Applying UTM to Ireland results in co-ordinates that have a 7-digit northing and 6-digit easting, compared to the current IG reference system, which has 6 digits in each.
|Central Meridian||9° West|
|Scale on CM||0.999 600|
|0° 00′ North
9° 00’ West
|False Origin (metres)||500 000 W