How do I give a six-figure grid reference?

The Ordnance Survey National Grid divides Great Britain into 100-kilometre squares, each with a two-letter code, and then into one-kilometre squares printed as the blue grid on every OS map. A grid reference is simply a way of naming a point within that structure, and the rule for reading one has not changed in decades: along the corridor, then up the stairs.

Safety

What you'll need

• An OS map with the grid printed on it
• A romer or the scale on a compass baseplate

Step by step

1. Find the two-letter code for the area.It is printed in the map legend and often faintly in the corners of the grid. It identifies the 100km square — SD, NY, SH, NN and so on. Without it, your reference is ambiguous nationally.
2. Read the easting: go along the corridor.Find the vertical grid line immediately to the left of your position and read its two-digit number, printed at the top and bottom of the map. That is the first pair of digits.
3. Estimate the tenths eastwards.Judge how far your position lies across the square from that line, in tenths. That single digit is the third figure. A romer scale on a compass baseplate does this accurately; by eye is fine for a 100-metre square.
4. Read the northing: then up the stairs.Find the horizontal grid line immediately below your position and read its two-digit number from the side of the map. That is the fourth and fifth figures.
5. Estimate the tenths northwards.How far up the square your position sits, in tenths. That is the sixth figure.
6. Assemble it in the right order.Letters, then three easting digits, then three northing digits: SD 123 456. Never mix the order — the whole convention is eastings first.
7. Use eight figures if you need ten-metre precision.Four digits for the easting and four for the northing, estimating hundredths of the square. This needs a romer and is what a mountain rescue team will ask for if they can get it.
8. Say it out loud correctly.Read the letters using the phonetic alphabet — Sierra Delta — then the digits individually: one two three, four five six. Never say a hundred and twenty-three.
9. Know how to get one from a phone as a backup.OS Maps, OS Locate and several other apps read out a full grid reference from GPS, and the free what3words system is accepted by UK emergency services as an alternative. Learn to find both before you need them, and remember the phone may be dead or without signal, which is exactly why you can also do it from paper.

Tips

• The mnemonic that survives is: along the corridor, then up the stairs. Eastings first, always.
• A compass with romer scales for 1:25,000 and 1:50,000 turns a rough estimate into an accurate six or eight-figure reference in seconds.
• In an emergency in the UK, dial 999, ask for the police, then ask for mountain rescue or coastguard as appropriate — and have the grid reference and the two letters ready before you call.
• If you have no signal, in most of the UK you can text 999 provided you have registered the phone in advance. Register today; it takes two minutes.

Common mistakes

• Giving six figures without the letters — Six-figure references repeat every 100km. SD 123 456 and NY 123 456 are hundreds of kilometres apart, and giving the number alone can send help to the wrong one.
• Reading northings first — It produces a valid-looking reference for a completely different place. Eastings always come first.
• Using the grid line above or to the right of your position — The convention is the line below and to the left — the bottom-left corner of the square you are in.

Questions people ask