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Ordnance Survey FAQ and Excel Mapping Routines

The OS Cartographic System


The Ordnance Survey of Great Britain is a Transverse Mercator projection.  This is a particularly ideal choice for a tall thin area like Britain (or Chile!) in that distances, areas and angles are well mapped with minimal error.   Whilst Transverse Mercator is also used in USA and USSR and for UTM it has limitations when used for areas with large east/west distances (like the former Soviet Union) which force the cartographer to segment the area with discontinuities which are quite absent with the OS system across Britain.

The OS projection is based on a true origin at 49 degrees North, 2 degrees West which is near the Channel Islands.

From this origin, a square grid is imposed on top of the ‘converging’ lines of longitude and the ‘curved’ lines of latitude.  Points are located by measuring distances in metres east and north from this true origin.  Since this process will introduce distortion as you move away from the central longitude of 2 degrees west, a scale factor of 0.9996012717 is introduced which reduces this distortion – two lines of longitude with zero distortion are thus created.  Incidentally, this is very slightly different to the factor of 0.9996 used for UTM.  Furthermore, to eliminate the inconvenience of negative values of easting, the concept of a false origin is introduced.  A value of 400000 (400 Km) is added to eastings and 100000 (100 Km) subtracted from northings so placing this false origin near the Isles of Scilly off Lands End in the south west.   The model uses the Airy spheroid, OSGB of 1936.  Readers wishing to delve very deeply into the topic of map projections are referred to a publication by a former colleague at the University in Swansea, South Wales:

Coordinate Systems and Map Projections, D H Maling,  Pergamon Press, ISBN 0-08-037234-1.

See also the OS web site

Phil Brady © 2020