## Understanding Ordnance Survey Grid References

The Ordnance Survey (OS) is Britain's national mapping agency. It publishes maps of England, Wales and Scotland at a variety of different scales for both professionals and the public. The most popular OS maps are the Landranger series.

### What is the Landranger Series of Maps?

The Landranger series is a set of 204 maps covering all of England, Wales and Scotland, including the Hebrides and Shetland Isles. The maps are drawn at a scale of 1:50,000 so that 2 centimetres on the map corresponds to one kilometre on the ground, or roughly 1 inch to a mile. Each map covers an area of 40 kilometres by 40 kilometres (roughly 25 miles by 25 miles). The maps include all roads, paths, footpaths, public rights-of-way, railways, rivers, lakes, canals and churches. Height is shown by contours at 10 metre (33 foot) intervals.

Along the edges of the map, latitude and longitude are marked at intervals of one minute of arc, but every map is also overlaid by a network of one-kilometre squares which make up the British National Grid. The grid squares are numbered along all four edges of an OS map. The British National Grid provides a much simpler way to specify the position of any feature on an OS map, from a city down to the smallest farm or church or stream.

### How Does the British National Grid Work?

The easiest way to understand the British National Grid is to picture a map of England, Wales and Scotland with a network of one-kilometre squares laid over it, like a sheet of graph paper. Then the position of any place in Britain can be specified by its coordinates on the graph - its distance east and north of a zero point which lies off the south-west coast of Cornwall.

To simplify the system, there is a grid of larger squares, each 100 kilometres by 100 kilometres (roughly 60 miles by 60 miles), which each have a two-letter designation such as TQ, SV, NA or HZ. Square SD, for example, covers much of Cumbria, as well as parts of northern Lancashire and and the western Yorkshire Dales. The position of any place within that square can then be specified by its distance east and north of the south-western (bottom-left) corner of the square.

#### Example 1: Windermere

Windermere is a small town on the eastern shore of Lake Windermere in the English Lake District. Its grid reference is SD4198. That places it in the 100-kilometre square SD. The remainder of the reference should be an even number of digits. This is split into two halves of equal length. In this example, this gives a pair of two-digit numbers, 41 and 98.

The first number, 41, is called the easting. It tells you how far east (to the right on the map) to move within square SD to find Windermere.

The second figure, 98, is called the northing. It tells you how far north (upwards on the map) to move to find Windermere.

So we will find Windermere by moving 41 kilometres east and 98 kilometres north of the bottom-left corner of square SD.

#### Finding Windermere on a Landranger Map

To find Windermere on a Landranger map, first choose the correct sheet. Sheets 96 and 97 overlap and Windermere appears on both of them. Open the sheet out and read off the easting from the blue numbers that appear on the top and bottom edges of the map. Find the vertical (north-south) line labelled 41. Windermere lies just to the right of this line.

Then read off the northing from the blue numbers that appear on the left and right edges of the map. Find the horizontal (east-west) line labelled 98. Windermere lies just above this line.

Thus Windermere (or part of it) lies in the one-kilometre square whose left-hand edge is the vertical line labelled 41 and whose bottom edge is the horizontal line labelled 98.

#### Example 2: High Greaves Farm

A grid reference such as SD4198 specifies the position of a place to the nearest kilometre. This is usually accurate enough for large features such as cities, towns and lakes, but to find smaller features such as villages or churches or individual farms, a more detailed grid reference is needed. This is called a six-figure reference and it allows you to pinpoint a feature to within 100 metres. High Greaves Farm has a six-figure grid reference of SD238781 and it appears on Landranger sheets 96 and 97.

#### Finding High Greaves Farm on a Landranger Map

To find High Greaves Farm on a Landranger map, we split the numerical part of the grid reference into an easting of 238 and a northing of 781.

First use the easting to find the vertical lines labelled 23 and 24. Then use the northing to find the horizontal lines labelled 78 and 79. High Greaves Farm lies inside the one-kilometre square bounded by these lines.

In an east-west direction, it is 8/10 of the way between the vertical lines labelled 23 and 24, so it is closer to line 24.

In a north-south direction, it is 1/10 of the way between the horizontal lines labelled 78 and 79, so it is closer to line 78.

### Conversion to Latitude and Longitude

If you prefer latitude and longitude, convert the grid references here. The easting and northing must be given to three figures each.
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