## 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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