A grid reference is how you turn "somewhere on the hill" into a point somebody else can walk to. It is the most practical five minutes of learning in this domain, it is easy, and almost everybody who has not been taught it gets the two numbers the wrong way round.
What the grid is
The Ordnance Survey National Grid covers Great Britain in a set of squares.
- 100 km squares, each identified by two letters: SD, NY, SU, TQ and so on.
- Within each, numbered lines at 1 km intervals, which are the blue grid lines on the map.
- The numbers run west to east and south to north, increasing in both directions.
Eastings are the lines running up the map, numbered along the top and bottom, and they measure how far east you are. Northings are the lines running across the map, numbered up the sides, measuring how far north.
The mnemonic is "along the corridor and up the stairs": you go along first, then up. Eastings always come first.
Reading a reference
- Find the vertical grid line to the LEFT of your position. Read its two-figure number. That is your easting.
- Find the horizontal grid line BELOW your position. Read its two-figure number. That is your northing.
- Put them together. Easting first. That is a four-figure reference and it describes the one kilometre square you are in.
To get six figures, imagine each square divided into ten by ten.
- Estimate how many tenths across the square you are, from the left line. Add that digit to the easting.
- Estimate how many tenths up the square you are, from the bottom line. Add that digit to the northing.
- Read it as three digits then three digits.
To get eight figures, estimate to hundredths rather than tenths, which in practice means judging quarters and eighths of the square by eye.
A romer helps a lot. Most compasses have small scales on the baseplate marked for 1:25,000 and 1:50,000, and laying one on the map gives the tenths directly rather than by estimate.
How precise to be
Six figures is the common habit and it is not enough for a casualty. A hundred metre square on open moor in cloud is a real search, and the people searching it are working in the same weather you are.
Give eight when it matters. Estimating to the nearest eighth of a grid square is easily done by eye and it shrinks the area they must cover by a hundredfold.
The two letters
Always give them, and give them first.
Without the letters the numbers repeat every hundred kilometres across the country. A six-figure reference with no letters describes several dozen locations in Great Britain, and there is a real history of confusion caused by leaving them off.
Find them on the map, in the margin, in the grid reference explanation panel, and usually printed faintly across the map face.
Say them clearly, ideally with the phonetic alphabet: Sierra Delta rather than SD, since the letters are easily confused over a poor phone line or a radio.
Giving a position without a map
You may have no paper map, or be somewhere off the edge of the one you have.
- what3words gives a three word address for every three metre square, and it is accepted by UK emergency services. It works offline once the app is installed, which is worth doing in advance rather than at the point of need.
- Latitude and longitude from a phone works and is slower to say. Give the format you are reading, and read it digit by digit.
- The OS Locate app gives a full grid reference from the phone's own position, offline, without any map. It is free, small, and among the most useful things to have installed before a problem.
- A described position is better than nothing and can be very good: "on the east bank of the river, about 400 metres upstream of the stone bridge on the B-road, by a ruined sheepfold." Distances, directions and named features.
- Take a photograph of where you are before you go for help, if you are leaving a casualty. It gives you and the rescuers something concrete.
A phone can give a position when it cannot make a call. GPS receives from satellites and needs no network at all, so the location works even with no signal and, if you have downloaded offline maps in advance, so does the map.
Calling for help
Text to 999
Worth setting up before you need it, and almost nobody has.
Emergency SMS lets you text 999, which works where there is too little signal for a call and is the standard route for deaf and speech-impaired users.
It only works from a registered phone, and registration must be done in advance: text the word register to 999 and follow the reply. It takes a minute and it cannot be done at the moment you need it.
Do this for every phone in the household, today. It costs nothing and there is no downside.
When texting, give the same information in the same order, and keep it short and clear. Expect a reply and wait for it.
Practising it
The whole skill takes half an hour to learn and never leaves.
- Find your own house on a 1:25,000 map and give its eight-figure reference. Check it against the OS Locate app standing in the garden.
- Take a reference from the map and walk to it, and see how big a hundred metre square really is on the ground. That single exercise is what convinces people to give eight figures.
- Teach it to everybody in the household who might be the one making the call, including older children. It is a genuinely useful thing for a twelve year old to know.
- Say one out loud with the phonetic letters, slowly, and have somebody write it down and read it back. Doing it once shows you how easily digits are misheard.