Coordinate Reference Systems used in United Kingdom

The British National Grid (EPSG:27700) is the projected CRS used by Ordnance Survey for all of Great Britain — every OS map, the Land Registry and most local-authority GIS. The underlying datum is OSGB36 (EPSG:4277), based on the Airy 1830 ellipsoid. For sub-metre work converting between OSGB36 and WGS 84 requires the OSTN15 grid-shift transformation. Northern Ireland uses the Irish Grid (EPSG:29900-series) historically and the Irish Transverse Mercator on ETRS89 today.