Coordinate Reference Systems

The earth is an uneven sphere, and in order to present it in two dimensions on a computer screen, we need to use a particular projection, based on a specific ellipsoid and datum, to model the shape of the world. The projection, ellipsoid and datum together are often known as the Spatial Reference System (SRS) or Coordinate Reference System (CRS) of the map. We use two main Coordinate Reference Systems in this project, the British National Grid and Global Spherical Mercator projection.

British National Grid

Within the UK, the British National Grid is often easiest to use, and the primary system used in Ordnance Survey gazetteers and mapping. Further information about the British National Grid is available from Ordnance Survey.

Main parameters:

  Map projection:Transverse Mercator
  Ellipsoid:Airy 1830
  EPSG Code:EPSG: 27700

Further information on its specification in different formats.

Global Spherical Mercator

Internationally, the Global Mercator projection on a spherical approximation of the Earth (usually termed the Spherical Mercator) is widely used (eg. in Google Maps and Earth, Microsoft Bing, Yahoo Maps). You often need to convert to use this system when creating a georeferenced overlay using Openlayers or in a Google map mashup.

Main parameters:

  Map projection:Spherical Mercator
  EPSG Code:EPSG: 4326(for coordinates in latitude/longitude)
  EPSG Code:EPSG: 900913(for coordinates in metres)

Further information on its specification in different formats.

The standard latitude/longitude projection based on the WGS84 datum is “EPSG:4326”. “EPSG:4326” describes latitude/longitude coordinates. The string “EPSG:900913” is newer, and not supported by all software, but describes coordinates in meters in x/y.

Converting between Coordinate Reference Systems using the EPSG codes

The European Petroleum Survey Group or EPSG (1986 – 2005) was a scientific organization with ties to the European petroleum industry. Its role from 2005 was taken over by the Oil and Gas Producers Surveying & Positioning Committee. The EPSG compiled and disseminated the EPSG Geodetic Parameter Set, a widely used database of geographic and projected coordinate systems.

The EPSG codes often provide an easy method of transforming a map between different Coordinate Reference Systems, for example, using the gdaltransform program. Further information on gdaltransform and other gdal utility programs.

It is also easy to transform between coordinate systems opening or saving files in ArcGIS and QGIS. For further information, see Specifying a coordinate system in ArcGIS, and in QGIS, view Settings > Project Properties, or Raster > Assing Projection and Chapter 6 of the QGIS manual.

Further information: