British National grid (audit)

The most common and most accurate CRS in use in the UK is the British National Grid OSGB 36 (EPSG:27700, accurate to 1m).

British National Grid references are given in two formats:

Alphanumeric: ST 59828 62724

Easting / Northing: 359828 162724

In the heritage sector we often use the alphanumeric system, both as co-ordinate references but also as part of our labelling structure for records and archives. An example of this is the OS index cards that formed the first Sites and Monuments records, (the precursors to our Historic Environment Records) where the Antiquity Number, the unique reference for the site, contains the National grid number of the 1:10,000/10,560 sheet within which the site falls.

Co-ordinates derived from and used by GIS are natively fully digital eastings and northings so we need to understand how to convert alphanumeric references to easting / northing and vice versa.

Image of an OS Index Card, with Antiquity Number shown top right (from Gilman, P 2004 Sites and Monuments Records and Historic Environment Records in England: is Cinderella finally going to the Ball?, Internet Archaeology 15:3 https://intarch.ac.uk/journal/issue15/3/toc.html)

Alphanumeric reference grid for England, Scotland and Wales. Each letter can be replaced by a number (in brackets) that represents the first digit lower left corner of that 100km square.

This video explains how the OS alphanumeric references show accuracy and how to convert them to eastings / northings for use in GIS. There is a link below to some practice exercises to embed your knowledge.

Converting co-ordinates practice sheet
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