GRID REFERENCES

National Grid Lines

The whole of Great Britain is divided into squares of 100 km, and each square is given a prefix of two letters. These letters are found on the map and are coloured blue.

When you quote a six figure grid reference you should put the two letters of the map you are using in front of the grid reference.

For example you may have a grid of 509582 in SW Scotland. The grid reference you should quote would be NX509582

Without the letters the grid reference 509582 would be repeated in every 100km square.

There is another way to quote grid references:

For example Sheet 55, Grid 903102
Or Sheet 55, Lochgilphead, Grid 903102
All these are acceptable

National Grid

Showing where they join

Eastings and Northings
These are the grid lines that are marked on the map and numbered in the margins both are marked in blue.
Eastings are the vertical lines on the map.
Northings are the horizontal lines on the map
To help identify individual squares and to give or plot grid references, the lines are numbered.
Eastings start with a low number on the West edge of the map, with the numbers getting higher to the East edge of the map.
Northings, low number at the South edge of the map getting higher to the North edge of the map.
Hence the terminolgy Eastings and Northings.
The numbering of the eastings and Northings always starts from 00 to 99 to 00 this change over will appear on some maps but not all of them. Remember a 1:50,000 Ordnance Survey map measures 40 x 40 grid squares.

Some Rules For Giving and Plotting Grid References

Windmill, Windpump and Lighthouses - Bottom centre.
Buildings - Centre.
Public Telephones,CampSites - Where the arrow or black line points to not the symbol.

Precision of Grid References Using the estimation by eye of tenths of a grid square the accuaracy that can be expected is about plus or minus one tenth of a grid square. At a scale of 1:50,000, a grid reference may be given to the equivalent of plus or minus 100 metres on the ground.

If greater precision is required a Romer must be used to measure the tenths within the grid square. The expected accuracy would then be plus or minus 50 metres.

Four figure grid reference

Six figure grid reference

Four Figure Grid Reference

Eastings first part 70 - Northings 48, so the four figure grid ref would be NS7048, or sheet number followed by the grid.

To use a four figure grid reference you must mention something in that grid sq as a reference point. But remember if there are more than one of the same objects in that grid square it may get confusing

For example: Meet me at the church in grid sq NS7048, but there must only be one church in the grid sq.(see example above).

Six Figure Grid Reference

Eastings 686 - Northings 566. Full six figure grid reference would be the picnic area at grid NS 686566. (see example above).

The Romer

The romer provides a means of interpolating the last digits of eastings and northings of a grid reference. They are usually incorporated in other instruments such as protractors and compasses, also some maps have romers printed in the margin.(See compass page).

Romer Scales

The two scales ,which are used the most on the compass, are 1:25,000 and 1:50,000

Click here to go to Silva Compasses

Romer

How to use the romer

USE OF THE ROMER

Choose the correct scale of romer for the map you are using. Place the corner of the romer on the feature or object you want a grid reference of, in this case the path and road junction.

Locate the easting and northing coordinates in this case 77 and 34.

Then look and see which one of the lines on the romer (tenths) is near or touching the relevant grid lines, then count how many tenths for each coordinate.

The full six figure grid reference in this case would be 773342.

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