Coordinate reference system

Coordinates in the OS DPN product are provided in three dimensions.

The Geography Markup Language (GML) and GeoPackage product formats enable the use of the British National Grid (BNG) coordinate reference system for the planimetric element of the geometry. The BNG spatial reference system uses the OSGB36 geodetic datum and a single Transverse Mercator projection for the whole of Great Britain. Positions on this projection are described using easting and northing coordinates in units of metres. The BNG is a horizontal spatial reference system only; it does not include a vertical (height) reference system.

The height element is provided using the same datum (i.e. Ordnance Datum Newlyn) used for other Ordnance Survey products that contain height.

In the GML data, this combination of BNG and Ordnance Datum Newlyn is represented by reference to its entry in the EPSG registry, as https://epsg.io/7405.

The vector tiles product format is in the Web Mercator (EPSG:3857) projection. This projection is a global coordinate reference system.

Using the data with GPS devices

GPS devices may only provide coordinates using the WGS84 coordinate reference system (EPSG code 4979, although 4326 is often used also). The OS DPN data is supplied using a combination of coordinates in the OSGB36 BNG system and heights relative to Ordnance Datum Newlyn (EPSG code 7405) for both the GML and GeoPackage formats.

To allow the data to be used with positions from GPS devices, a transformation is required. The transformation can be applied to the OS DPN data (to move it on to WGS84) or to the GPS coordinates (to provide positions in OSGB36 BNG). The choice may depend on the coordinate systems of any other datasets in use.

Given the difference that can ensue from using the incorrect coordinate reference system, care must be taken to use the appropriate transformation. A recommended transformation is the seven parameter one published in A Guide to Coordinate Systems in Great Britain (PDF download). This transformation is EPSG code 1314.

Further information on coordinate systems and transformations is available on the OS website.