Ordnance Survey data and GPS measurements

All map data contains coordinates which describe the location of features in a coordinate reference system. All Ordnance Survey data available through Digimap uses the National Grid Reference system to give coordinates of features. The technical name for the National Grid coordinate system is OSGB36 National Grid. The OSGB36 part of the name, which stands for Ordnance Survey Great Britain 1936, refers to the geodetic datum used; the National Grid part refers to the map projection and grid referencing convention for eastings and northings.

The following are used interchangeably, although in reality they mean slightly different things:

British National Grid = BNG = OSGB36 = EPSG:27700

Users may wish to collect their own data using a Global Positioning System (GPS) receiver, and merge the resulting data with Ordnance Survey data. GPS data is measured in a different coordinate reference system from OSGB36, which is known as WGS84 (World Geodetic System 1984), and for most purposes is equivalent to ETRS89 (European Terrestrial Reference System 1989).

In order to be able to merge data from GPS with data from Ordnance Survey, the two sets of data need to be using the same reference system. One way of achieving this is to transform the GPS data from WGS84 or ETRS89 into OSGB36.

Some GPS equipment has such a transformation built in. Some GPS manufacturers supply additional software to allow users to carry out this transformation. Ordnance Survey have created a dedicated GPS website: http://www.ordnancesurvey.co.uk/oswebsite/gps/ explaining the technical issues involved, and including a free high-accuracy Coordinate Convertor between ETRS89 and OSGB36. This Coordinate Convertor allows conversion in both directions (ETRS89 to OSGB36, and OSGB36 to ETRS89) for either single points, or for a set of points by submission of a text file. The accuracy of the conversion is 20cm.

The Ordnance Survey's GPS site includes instructions for use.