Compass Basics for Drivers

The needle swings as you walk around the truck. Or you follow a bearing and the map says you're somewhere else entirely. Both happen, and both have clear causes.

First: what does the needle point to?

A compass has a magnetized needle that pivots to align itself with magnetic north.¹ Magnetic north is the direction the north end of the needle points. It isn't the same as true north, which is the direction along a line toward the geographic North Pole.²

Problem: the needle won't settle or points oddly

Magnetic compasses are influenced by any fields other than Earth's. Sources include large iron or steel bodies, electrical engines and strong permanent magnets.¹ A vehicle fits that description.

So try this, in order:

1. Step well away from the vehicle, including the bonnet or hood and the roof rack.
2. Keep phones, speakers and magnetic mounts away from the compass.
3. Hold the compass level so the needle can swing freely.
4. Wait for the needle to stop moving.

A compass is also subject to errors when it is accelerated or decelerated in an automobile.¹ So read it with the vehicle stopped, and well away from the metal and the engine.

What a bearing is

A bearing is a direction given as an angle. Absolute bearing refers to the clockwise angle between north and the thing you're pointing at. For example, an object due east has an absolute bearing of 90 degrees.³

You count clockwise around the dial, starting with north at zero. Use that to sense-check any reading. If you aim at a hill that's clearly east and the compass reads close to zero, something's wrong.

Problem: the map and the compass disagree

This is usually declination. Magnetic declination is the angle between magnetic north and true north at a particular location.² The needle points to magnetic north, so check which north your map uses.

Two things to know:

• Declination isn't the same everywhere. It depends on where you are.
• It changes over time, so an old map's figure may be out of date.²

By convention, declination is positive when magnetic north is east of true north.² Many maps print the local figure in the margin with an arrow diagram. Read the margin, check its date, and apply the correction the way your compass maker and map explain.

Some orienteering compasses have an adjustable declination setting.¹ Follow the maker's instructions for your own model.

Using a compass with a map

The compass doesn't replace the map. It adds a direction. Think of the two as a pair: the map shows what's out there, and the compass tells you which way to face.

• To find a direction on the map: read the map's bearing between two points, adjust for declination, and turn that into a direction on the ground.
• To check where you are: take a bearing on a landmark you can see, then draw that line on the map. Your position is somewhere along it.
• To confirm a track: if the map says the road heads northeast, check the compass agrees before you pick a fork.

A second bearing on a different landmark gives a crossing point, which narrows down where you are.

Know the limits

A compass is a back-up. It tells you which way, not how far. It can't see a cliff or a washed-out creek either, so keep to tracks you've checked and stay within the limits of your vehicle and your skills.

Practise in a park first. Learn how your own compass behaves, and read the maker's guide. For distances, see map scale and grid references. For contour lines and symbols, try reading a topo map.