The Compass
Map Reading and Navigation
Course 1 · Chapter 4 · The Compass: Direction You Can Measure
A map tells you what is around you; a compass tells you which way to go. The two together let you walk to a place you cannot yet see. This chapter introduces the baseplate compass, explains how directions are written as bearings from 0 to 360 degrees, and untangles the three different norths you will meet: true, grid and magnetic.
The Baseplate Compass
Walkers normally use a baseplate (or orienteering) compass: a clear plastic plate with a rotating housing and a magnetic needle inside. Its parts are worth learning by name.
Magnetic needle
A magnetised needle that swings to point towards magnetic north. The red end is the north end.
Rotating housing (bezel)
The ring around the needle, marked in degrees from 0 to 360 with the main points of the compass. You turn it to set a bearing.
Orienting arrow and lines
An arrow (often outlined) painted on the floor of the housing, with parallel orienting lines. You line the red needle up with it, so it works like a fixed north marker.
Direction-of-travel arrow
An arrow on the baseplate, pointing away from you. It shows the way to walk once the compass is set.
Index line
The mark at the base of the direction-of-travel arrow where you read the bearing off the housing.
Edge and scales
The straight edge of the plate has ruler scales (often for 1:25,000 and 1:50,000) for measuring distances and reading grid references.
What Is a Bearing?
A bearing is a direction written as the angle, measured clockwise from north, between north and the line you are interested in. A full circle is 360 degrees, so a bearing is always a number from 0 to 360, normally written with three digits and a degree sign, for example 045 degrees.
| Direction | Bearing |
|---|---|
| North (N) | 0 or 360 degrees |
| North-east (NE) | 45 degrees |
| East (E) | 90 degrees |
| South-east (SE) | 135 degrees |
| South (S) | 180 degrees |
| South-west (SW) | 225 degrees |
| West (W) | 270 degrees |
| North-west (NW) | 315 degrees |
The back bearing is the direction in which you would look back along the line you came. It is always exactly opposite: add 180 degrees to the bearing, or subtract 180 if the bearing is already above 180. It helps you check that you are not walking the wrong way.
Three Norths
Unfortunately there is more than one north, and a navigator must know which one is meant.
| North | What it is | Where you see it |
|---|---|---|
| True north | The direction of the geographic North Pole, where the lines of longitude meet | Used in astronomy and for long-distance navigation |
| Grid north | The direction of the vertical grid lines on the map (the northings run towards it) | All bearings you measure on the map are grid bearings |
| Magnetic north | The direction a compass needle points, towards the Earth's magnetic pole | All bearings you read in the field are magnetic bearings |
Grid north and true north are close together over most of a map and can be ignored for walking. The one that matters is the gap between grid north and magnetic north, called the magnetic variation (or declination). It is printed in the map key together with the year it applies to, and with how fast it changes each year. It is not the same everywhere, and it changes slowly over time because the magnetic pole moves. In recent years the variation in much of Britain has been a small number of degrees west of grid north, but always read the figure for your own sheet (check).
Converting Between Grid and Magnetic
When the magnetic variation is west of grid north (the common case in Britain in recent years, check your key), the rule is:
- Grid to magnetic: add the variation. (Mag is more, so grid to mag: add.)
- Magnetic to grid: subtract the variation.
If the variation is east the rules swap. Always wrap the answer back into the range 0 to 360. Many compasses have an adjustable declination scale that lets you build the correction into the setting so you do not have to do the arithmetic every time.
Common Mistakes
| Mistake | Fix |
|---|---|
| Following the needle instead of the direction-of-travel arrow | The needle only points to north; you walk where the travel arrow points |
| Ignoring the magnetic variation | Read the key and apply it, or use the compass's declination scale |
| Adding when you should subtract (or the reverse) | Note whether the variation is west or east, then apply the rule |
| Forgetting to wrap round 360 | Add or subtract 360 so the answer lies from 0 to 360 |
| Reading the needle near metal | Step away from metal objects and recheck |
| Using an out-of-date variation figure | Check the year printed in the key and allow for the yearly change |
Challenges
(a) Give the bearings of north, east, south, west and south-west. (b) Find the back bearing of 60, 300 and 175 degrees. (c) You are facing 350 degrees and want to face 020 degrees. Which way and how far do you turn by the shortest route?
The magnetic variation is 2 degrees west of grid north. Convert (a) a grid bearing of 90 to magnetic, (b) a magnetic bearing of 90 to grid, (c) a grid bearing of 1 to magnetic, (d) a magnetic bearing of 1 to grid. Explain what you do when the answer is below 0 or above 360.
Write a short program or spreadsheet that (a) turns a bearing into the nearest of the 16 compass points (N, NNE, NE ...), (b) gives the back bearing, and (c) converts grid to magnetic and magnetic to grid for a given westerly variation. Test it with bearings 0, 45, 100, 200 and 350, and with the conversions from Exercise 2.
Quick Reference
- Bearing: clockwise angle from north, 0 to 360 degrees
- Back bearing: add or subtract 180
- Map bearings: measured from grid north
- Compass bearings: measured from magnetic north
- Variation west: grid to magnetic add, magnetic to grid subtract
- Always: read the variation from the key, and keep metal away