Ordnance Survey Maps - British Grid Reference System

British Grid Reference System

The British Grid Reference System is based on the OSGB36 TM datum (Ordnance Survey Great Britain 1936), and can be used to accurately pinpoint any location in Great Britain and it‘s outlying islands through the use of a unique Ordnance Survey map reference.

Locations in the Republic of Ireland and Northern Ireland can be pinpointed in a similar way using the Irish National Grid Reference System.

Map of the UK showing the two letter grid structure Ordnance Survey map references for locations in Great Britain are usually given as:

[2 Grid letters]  [3 figures(easting)]  [3 figures(northing)]

But can also be given as:

[6 figures(easting)], [6 figures(northing)]

To arrive at a unique Ordnance Survey map reference, Great Britain is first divided into a series of 500km squares starting at the Southwest corner of the country. Each of these 500km squares is allocated a single reference letter (S, T, N, H or O).

Each 500km square is then subdivided into 25 squares, 100km by 100km. Each of these 100km squares is allocated a reference letter (A to Z, omitting I, starting with A in the north-west corner of the parent 500km square).

In this way each 100km square can be referred to by a unique 2 letter reference, with the first letter referring to the parent 500km square, and the second letter referring to a particular 100km square within it.

The approximate extent of the resultant 100km grid is illustrated in the map of the UK displayed on the right.

Map of Wales showing the 100km grid structure Wales is covered by eight of these 100km squares, as shown in the map of Wales on the left.

The Ordnance Survey grid reference for any location in Wales should therefore start with one of the following: This indicates within which particular 100km square that particular location can be found.

Much greater accuracy is of course required if the British Grid Reference System is to be of any practical value when trying to pinpoint a particular location on an Ordnance Survey map.

Each two letter 100km square is therefore further subdivided by vertical and horizontal grid lines, at 10km intervals, to produce one hundred 10km squares.

Map showing subdivision of 100km Square SN The vertical line forming the left hand side of the 100km square is labelled zero, and each successive vertical grid line within the 100km square is labelled in kilometers.

The horizontal line forming the bottom of the 100km square is also labelled zero, and each successive horizontal grid line within the 100km square is labelled in kilometers.

Any intersection of a vertical and horizontal line within square SN is therefore so many kilometers east of that square‘s zero point (the easting), and so many kilometers north of that same zero point (the northing).

The centre point of square SN could therefore be said to have a grid reference of SN 50 50.

To further improve the accuracy of the British Grid Reference System, each 10km square is further subdivided into a hundred 1km squares.

Map showing subdivision of a 10km Square within 100km square SN The diagram on the left is an enlarged view of the four 10km squares in the bottom left hand corner of the SN square shown on the right.

The first 10km square is subdivided by vertical and horizontal grid lines at 1km intervals.

The centre point of this 10km square could be said to have a grid reference of SN 05 05.

But to accurately establish the grid reference of the adjacent black dot, we have to imagine that a further 10 x 10 grid has been superimposed on the 1km square within which the dot is located.

The black dot can then be seen to be aproximately 4.5km east of zero (easting), and 5.5km north of zero (northing) within grid SN.

The full Ordnance Survey grid reference is therefore SN 045 055.

Ordnance Survey Maps

Landranger maps are amongst the most useful Ordnance Survey maps published. These use a scale of 1:50 000 (2cm to 1km), and use the British Grid Reference System discussed above.

If only one 100km square is covered by a map, the two letter code for that square can usually be found in each corner of the map. If more than one 100km square is covered, the relevant two letter codes are usually clearly printed where the 100km squares meet.

Each 10km square shown on a map is further subdivided into one hundred 1 km squares. The squares are formed from blue grid lines, spaced 1 km apart.

The eastings are marked along the top and bottom edges of each map, and the northings down the sides of each map.

When submitting an XC claim please use the following convention for an Ordnance Survey Grid Reference (OS Grid Ref):
[2 Grid letters]  [3 figures(easting)]  [3 figures(northing)]

The Ordnance Survey Grid Reference (OS Grid Ref) for each of our flying sites can be found in the SE Wales Sites Guide. The guide also lists the reference number of Landranger map that covers each site, and the Latitude and Longitude of each flying site.

You can also view a Google online map showing the actual location of any given UK Grid Reference.

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