Editing Ordnance Survey National Grid (section)

== Grid letters ==
[[Image:British National Grid.svg|thumb|100km squares]] [[File:Ordnance Survey 1-250000 - TF.jpg|thumb|Grid square TF. The map shows [[The Wash]] and the [[North Sea]], as well as places within the counties of [[Lincolnshire]], Cambridgeshire and [[Norfolk]].]]
The first letter of the British National Grid is derived from a larger set of 25 squares of size 500&nbsp;km by 500&nbsp;km, labelled A to Z, omitting one letter (I) (refer diagram below), previously used as a military grid.<ref>{{cite web|url=http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidetonationalgrid/page9.html|title=The grid square convention 1 (Guide to the National Grid, page 9)|url-status=dead|archive-url=https://web.archive.org/web/20060517190658/http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidetonationalgrid/page9.html|archive-date=17 May 2006|website=Ordnance Survey}}</ref> Four of these largest squares contain significant land area within Great Britain: S, T, N and H.  The O square contains a tiny area of [[North Yorkshire]], [[Beast Cliff]] at {{gbmappingsmall|OV 0000}}, almost all of which lies below mean high tide.<ref>{{cite web |last1=Standing |first1=Peter |title=OV0000 a unique grid square at Beast Cliff |url=https://www.geograph.org.uk/article/In-Search-of-OV0000 |publisher=Geograph Project |access-date=11 June 2007 |date=2006 |archive-date=1 November 2017 |archive-url=https://web.archive.org/web/20171101115044/http://www.geograph.org.uk/article/In-Search-of-OV0000 |url-status=live }}</ref>
For the second letter, each 500&nbsp;km square is subdivided into 25 squares of size 100&nbsp;km by 100&nbsp;km, each with a letter code from A to Z (again omitting I) starting with A in the north-west corner to Z in the south-east corner. These squares are outlined in light grey on the "100km squares" map, with those containing land lettered. The central (2° W) meridian is shown in red.