Grid References

 

GRID REFERENCES

 

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 grid squares and to give and plot a grid reference, the lines are numbered.
  • Eastings start with a low number on the west edge of the map, with the numbering getting higher towards the east.
  • Northings, low numbers at the south edge of the map and getting higher to the northern edge of the map.

Hence the terminolgy eastings and Northings.

The numbering of the eastings and northings always start at 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.

Precision of Grid References

Using the estimation of tenths of a grid square the accuracy that can be expected is about plus or minus a 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.

Grid References 

The whole of Great Britain is divided into squares of 100 kilometres, and each square is given a prefix of two letters.

These letters are found on the map and coloured blue.

When you qoute a 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 qoute would be NX509582.

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

There is another way to identify the map you are using to give a grid reference and that is to qoute the map sheet number and even the title of the map.

For example: Sheet 55, Grid 903102.

or, Sheet 55, Lochgilphead, Grid 903102

All these are acceptable.

Four Figure Grid Reference

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

To use a four figure grid reference you must mention something in that grid square as a reference point. But remember if there are more than one object or feature in that grid square it may get confusing.

See example opposite: Meet me at the church in grid square NS7048, but there must be only one church in that grid square.

Six Figure Grid Reference

See example below.

Eastings 686, Northings 566. Full six figure grid reference would be the picnic area at grid NS686566

 

Romer Scales

The two scales, which are used the most on the compass are, 1:25,000 and 1:50,000 and 1:40,000 on Harvey Super Walker maps.

Use of the Romer

Make sure when using a romer you choose the correct scale 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 eastings and northings in this case 69 and 01.

Then look and see which one of the lines are touching the relevant grid lines, then count how many tenths for each coordinate.

The full six figure grid reference for the road and path junction would be 696018.