Guidance on the choice of recommended coordinate reference system (CRS) in the UK Geospatial Data Standards Register for exchanging geospatial information
Overview of the CRS guidance
It is important to define which CRS you’re using when publishing data to keep the location information consistent and accurate across multiple applications. To do this you should use standards appropriate to the spatial extent of the data:
- use British or Irish National Grid for data related to the UK and Northern Ireland
- use ETRS89 (EPSG::4258) for Europe
- use WGS84 for worldwide geographic location points
You can also combine these standards and/or add other CRS to meet your organisation’s requirements, for example, when you need greater accuracy for precision engineering decisions.
This document should be referred to alongside the UK Geospatial Data Standards Register
CRS for Great Britain and Ireland
The CRS that should be used in Great Britain is the British National Grid. This is a planar coordinate system where the effects of earth curvature have been removed, allowing precise measurements to be made from geospatial data quoted in this system. A similar Irish Transverse Mercator system exists covering Ireland based on the European Terrestrial Reference Frame and should be used in Northern Ireland.
The register contains three CRS covering Great Britain and three covering Northern Ireland, split into 1D (height only), 2D (Easting & Northing only) and 3D versions (Easting, Northing & height).
CRS for use at the European Scale
The European Terrestrial Reference System 1989 (ETRS89) is a CRS that takes account of continuous plate tectonic movements and is fully compatible with Global Navigation Satellite System (GNSS) observations. This is achieved with the adoption of an epoch or point in time to remove the effects of tectonic motion. 1989 refers to the original realisation of this system from which all others are based.
The ETRS89 CRS is used extensively in the UK by GNSS users for positioning and mapping and is the basis of current OS and OS NI products. By using ETRS89 the UK is keeping its commitment to implementing the EU INSPIRE Directive.
The register contains one 2D and two 3D CRS listed, with the coordinates quoted using the latitude and longitude notation. The 3D versions are quoted with ellipsoidal height and with heights above mean sea level.
The Geocentric CRS in this section has its coordinate origin at the centre of the earth and is used for more specialist applications.
CRS for use at the global scale
The World Geodetic System 1984 (WGS 84) is a global CRS and an associated ellipsoid which enables the georeferencing of data on a worldwide scale for geodesy, mapping and navigation.
The WGS 84 / Pseudo Mercator CRS (also known as Web Mercator) is widely adopted in the UK, however it is not as rigorous in its definition. It is used across web applications for georeferencing, principally raster data and imagery. It should be used only for purposes having metre-level (or poorer) accuracy requirements.
The register contains a further two 2D and 3D WGS 84 CRS quoted using traditional latitude and longitude values with ellipsoidal heights. Two of these (one 2D and one 3D) are maintained by the EPSG for maritime and aeronautical navigation. The other two are maintained by the OGC and are used to support the GeoJson file format.
Finally there is a Geocentric CRS listed based on the WGS 84 ellipsoid, again used for maritime and aeronautical navigation.