Grid References
Latitude and longitude work anywhere on Earth. A national grid works better on the ground, because it is designed to be read off a paper map and said out loud without mistakes.
Why grids exist when we already have coordinates
Latitude and longitude are global and precise, but they are awkward in the field. They involve decimals or minutes and seconds, they need a hemisphere letter, and reading one aloud over a radio invites error. National grids solve a narrower problem: naming a point on this particular map, quickly, with digits only.
A grid overlays the map with numbered squares, usually one kilometre on a side. Every line has a number, and a reference is just the numbers of the lines you are next to, in a fixed order. The British Ordnance Survey national grid is the classic example, but almost every country has an equivalent.
Eastings and northings
The vertical lines are eastings, numbered left to right, so the number goes up as you move east. The horizontal lines are northings, numbered bottom to top, so the number goes up as you move north.
Eastings always come first. The old mnemonic is "along the corridor, then up the stairs": you walk along before you climb. Another is that it is in alphabetical order, E before N.
How to give a four-figure grid reference
Four figures name a one-kilometre square. It takes about five seconds once you have done it twice.
- Find the square Locate the grid square your feature sits inside.
- Read the easting Take the number of the vertical line on the LEFT edge of that square. Always the left, never the right.
- Read the northing Take the number of the horizontal line along the BOTTOM edge of that square. Always the bottom.
- Put them together Write the easting then the northing, with no space and no comma: 32 and 46 becomes 3246.
Six figures: dividing the square by ten
Four figures gets you to a one-kilometre square, which is enough to find a village but not enough to find a bridge. Six figures narrows it to about 100 metres, and the method is the same idea applied once more.
Imagine each grid square divided into ten by ten smaller squares. Estimate how many tenths across your point sits, and add that digit to the easting. Estimate how many tenths up it sits, and add that digit to the northing. You do not need to measure; eyeballing tenths is accurate enough, which is exactly why the system uses ten rather than a hundred.
Worked example
A church sits in the square with easting 18 and northing 72, about halfway across it and near the top.
- Easting 18, roughly 5 tenths across → 185
- Northing 72, roughly 8 tenths up → 728
- Eastings first, then northings
The six-figure reference is 185728.
How precise is each reference?
Each extra pair of digits divides the square by ten again.
| Reference | Example | Locates you to |
|---|---|---|
| Four figure | 3246 | A 1 km square |
| Six figure | 327463 | About 100 m |
| Eight figure | 32744635 | About 10 m |
| Ten figure | 3274946352 | About 1 m |
Six figures is the everyday standard for walking and for emergency services. Eight and ten figure references come from GPS rather than from reading a paper map.
Grid letters
Grid numbers repeat every hundred kilometres, so a bare six-figure reference is ambiguous across a whole country. National grids fix this with a letter prefix identifying the 100 km square: in Britain, a full reference looks like SU 327 463.
Within a single map sheet the letters can safely be dropped, because there is no ambiguity. Once you leave that sheet, or you are giving a reference to someone who might not have it, include them.
Check what you learned
Five questions on grid references. Pass at 70% or better to complete the lesson.
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Grid references, in one paragraph
A grid reference names a location using the numbered lines printed on a map. Vertical lines are eastings, numbered left to right, and horizontal lines are northings, numbered bottom to top. Eastings are always read first, which the mnemonic "along the corridor, then up the stairs" is meant to fix in memory. A four-figure reference such as 3246 combines the easting and northing of the bottom-left corner of a square and names a one-kilometre square. A six-figure reference such as 327463 mentally divides that square into tenths and adds one digit to each half, narrowing the location to about 100 metres. Eight and ten figure references add further pairs for 10 metre and 1 metre precision. Because grid numbers repeat every 100 kilometres, full national references also carry letters identifying the 100 km square, as in SU 327 463.
Common questions about grid references
How do you read a grid reference?
Read the eastings first, taking the number of the vertical line on the left of your square, then the northings, taking the number of the horizontal line along the bottom. Combine them with no space: easting 32 and northing 46 gives 3246. The mnemonic is "along the corridor, then up the stairs".
What is the difference between a four-figure and a six-figure grid reference?
Four figures name a whole one-kilometre grid square. Six figures divide that square mentally into ten by ten and add one digit to each half, giving a location accurate to about 100 metres. Six figures is the everyday standard for walking and for emergency services.
What are eastings and northings?
Eastings are the vertical grid lines, numbered so the value increases as you move east. Northings are the horizontal grid lines, numbered so the value increases as you move north. A grid reference is simply the easting followed by the northing.
Which corner of the square does a grid reference refer to?
The bottom-left corner. The reference names the square that lies above and to the right of the two lines you read. Using the top or right-hand lines instead puts you a kilometre out, and the wrong answer still looks like a valid reference.
What do the letters in a grid reference mean?
They identify which 100 kilometre square of the national grid you are in, because the numbers alone repeat every 100 km. In Britain a full reference looks like SU 327 463. Within one map sheet the letters can be dropped safely, but they should be included when the reference travels further.