Datum, ellipsoid and projection information

Shape and size of biaxial ellipsoids used in the UK

Name	Semi-major axis a (m)	Semi-minor axis b (m)	Associated datums and projections
Airy 1830	6 377 563.396	6 356 256.909[10]	OSGB36, National Grid
Airy 1830 modified	6 377 340.189	6 356 034.447	Ireland 65, Irish National Grid
International 1924 aka Hayford 1909	6 378 388.000	6 356 911.946	ED50, UTM
GRS80 aka WGS84 ellipsoid	6 378 137.000	6 356 752.3141	WGS84, ITRS, ETRS89.

The ellipsoid squared eccentricity constant e² is computed from a and b by equation (B1).

Transverse Mercator projections used in the UK

Projection	Scale factor on central meridian (F₀)	True origin, Φ₀ and λ₀	Map coordinates of true origin (metres), E₀ and N₀	Ellipsoid
National Grid	0.9996012717	lat 49° N long 2° W	E 400 000 N -100 000	Airy 1830
Irish National Grid	1.000035	lat 53°30′ N long 8° W	E 200 000 N 250 000	Airy 1830 modified
UTM zone 29	0.9996	lat 0° long 9° W	E 500 000 N 0	International 1924 or WGS84
UTM zone 30	0.9996	lat 0° long 3° W	E 500 000 N 0	International 1924 or WGS84
UTM zone 31	0.9996	lat 0° long 3° E	E 500 000 N 0	International 1924 or WGS84

EPSG codes for common datums and coordinate operations in GB

Many geographic information systems (GIS) and other methods of handling digital geographic data use unique identifiers - Spatial Reference System Identifier (SRID) - for datums, coordinate systems etc.. See https://en.wikipedia.org/wiki/Spatial_reference_system#Identifier. A commonly used SRID Authority is the EPSG (European Petroleum Survey Group) who maintain an online registry of SRIDs at https://epsg.org/home.html. The table below lists EPSG codes of ellipsoids, datums and coordinate operations in common use in the Great Britain.

Datum (and associated coord ref system) or operation name	EPSG SRID code
Airy 1830 ellipsoid	7001
GRS80 ellipsoid	7019
WGS84 ellipsoid	7030
OSGB36 Datum, 2D lat/long	4277
OSGB36 Datum, 2D National Grid	27700
OSGB36 Datum, 3D National Grid + ODN height	7405
ODN Vertical Datum	5701
ETRS89 Datum, 2D lat/long	4258
ETRS89 Datum, 3D lat/long/Height	4937
ETRS89 Datum, 3D XYZ	4936
WGS84 Datum, 2D lat/long	4326
WGS84 Datum, 3D lat/long/Height	4979
WGS84 Datum, 3D XYZ	4978
OSTN15 transformation, native 1km grid	7953
OSTN15 transformation, NTv2 version	7709
OSGM15 transformation, ODN	7711
OSGM15 transformation, Orkney Datum	7712
OSGM15 transformation, “Newlyn offshore”	7713
OSGM15 transformation, Lerwick Datum	7714
OSGM15 transformation, Stornoway Datum	7715
OSGM15 transformation, St. Marys Datum (Scilly Isles)	7716
OSGM15 transformation, Douglas Datum (IoM)	7717
WGS84 < - > OSGB36 Helmert transformation (section 6.6)	1314
ETRS89 lat/long + ODN height compound CRS	9424
ETRS89 lat/long + ODN (Offshore) height compound CRS	9425
ETRS89 lat/long + Orkney height compound CRS	9426
ETRS89 lat/long + Lerwick height compound CRS	9427
ETRS89 lat/long + Stornoway height compound CRS	9428
ETRS89 lat/long + Douglas height compound CRS	9429
ETRS89 lat/long + St. Marys height compound CRS	9430
British Isles height ensemble (approx. MSL height across whole of British Isles)	1288
ETRS89 lat/long + British Isles height ensemble compound CRS	9542