Grid References
Map Reading and Navigation
Course 1 · Chapter 2 · Grid References: Naming Any Place with Numbers
Describing a place in words ("the stile past the second farm") only works if both people know the area. A grid reference describes a place with a short number code that anyone holding the same map can decode. This chapter explains the grid, the rule for reading eastings before northings, and how to write and read four-figure and six-figure references. The examples follow the British National Grid used on Ordnance Survey maps; other countries use similar systems with different numbering (check the key of your own map).
The Grid
Printed over the map is a network of thin lines forming squares. Each line has a number printed on the map edge. On most walking maps the lines are 1 km apart, so each square is 1 km by 1 km on the ground. That is why the grid also helps with distance: two lines are always the same distance apart, however the paper is folded.
Eastings
The vertical lines. Their numbers rise as you move east (to the right). They are labelled along the top and bottom edges.
Northings
The horizontal lines. Their numbers rise as you move north (up the page). They are labelled along the left and right edges.
Grid square
The box between four neighbouring lines. A four-figure reference names one whole square.
Tenths
Splitting a square into ten parts in each direction lets you name a point within 100 m at 1:25,000 or 1:50,000.
Eastings First
Always give the easting (the number along the bottom) before the northing (the number up the side). The classic memory aid is "along the corridor, then up the stairs": you walk along before you climb. Reversing the two numbers names a different place, often many kilometres away, so this is the single most important habit in the chapter.
Four-Figure References
A four-figure reference names a whole square. Take the number of the vertical line on the left (west) side of the square, then the number of the horizontal line on the bottom (south) side. Join the two pairs into four digits.
| Step | What you do | Example |
|---|---|---|
| 1 | Find the square that holds the feature | A church |
| 2 | Read the easting of the line on its left | 34 |
| 3 | Read the northing of the line along its bottom | 72 |
| 4 | Write easting then northing | 3472 |
A four-figure reference is quick, but a 1 km square is large: a farm, a wood and a road junction might all sit in one square. It tells you where to look, not which feature you want.
Six-Figure References
To pin a point to within about 100 m, imagine the square divided into a 10 by 10 set of smaller boxes. Estimate how many tenths across the point lies from the left line, and how many tenths up from the bottom line. Then slot each tenth in after the matching two-digit line number.
Read the six figures as two halves: the first three are the easting, the last three are the northing. So 346723 means easting 346, northing 723. The third digit of each half is the tenth you estimated.
Reading a Reference
To go from a reference to a place, reverse the steps. Take 351698: easting 351 means line 35 and one tenth to the east; northing 698 means line 69 and eight tenths to the north. Find the line 35, move one tenth right, find line 69, move eight tenths up, and look where the two meet.
| Reference | Easting | Northing | Meaning |
|---|---|---|---|
| 3472 | 34 | 72 | Whole square, bottom-left corner at lines 34 and 72 |
| 346723 | 34 and 6 tenths | 72 and 3 tenths | A point within about 100 m |
| 351698 | 35 and 1 tenth | 69 and 8 tenths | Near the top of square 3569 |
Letters and the Wider Grid
The two-digit line numbers repeat every 100 km. On the British National Grid, two letters in front of the numbers (such as NT for the area around Edinburgh) say which 100 km square you are in. A full reference reads like "NT 346 723". Letters matter when you give a reference to someone far from the area, but for a day on one map you can usually leave them off if you say which map you are using.
Common Mistakes
| Mistake | Fix |
|---|---|
| Northing before easting | Along the corridor, then up the stairs |
| Using the line on the right or the top of the square | Always use the left and bottom lines |
| Counting tenths from the wrong side | Measure from the left line and the bottom line only |
| Treating 7 tenths as 0.7 of a different square | Tenths are digits 0 to 9, added after the two-digit line number |
| Reading the grid numbers off a different map | Line numbers differ from map to map, so check you are using the right sheet |
Challenges
A square has its left line numbered 52 and its bottom line numbered 08. Write the four-figure reference for the square. Then write the six-figure reference for a point 4 tenths across and 9 tenths up in that square, and a second point 0 tenths across and 5 tenths up.
Read these three six-figure references and describe each in words (line numbers and tenths): 218447, 603092, 470135. Then say how far, in kilometres, the first two squares' left and bottom lines are from each other if the grid lines are 1 km apart.
Write a short program or spreadsheet formula that takes a six-figure reference as a string and returns the two line numbers and the two tenths. Then extend it to return the distance in metres between two references on a 1 km grid, assuming each tenth is 100 m. Test it with 346723 and 351698.
Quick Reference
- Order: easting (along) then northing (up)
- Four figures: left line then bottom line; names a 1 km square
- Six figures: add tenths after each line number; names a point within about 100 m
- Reading: first three digits easting, last three northing
- Letters: name the 100 km square; add them for far-away readers
- Always: use the left and bottom lines of the square