Understanding and applying declination

The Earth behaves like a huge magnet whose magnetic poles are not exactly at the geographic poles. The compass needle lines up with the field lines of this magnetic field. Depending on where you stand on Earth, it therefore points somewhat east or west of true north. In some places the difference is almost zero, in others it is large.

Grid north is yet another direction: because the Earth is round and the map is flat, the grid lines do not all run exactly to the North Pole. For practical map-and-compass work, what matters most is the angle between your map's north lines and magnetic north. Many maps state exactly this value in the margin, often with a small diagram, a reference year and the annual change.

The magnetic field changes slowly. That is why declination shifts over the years too. An old map can show a value that is no longer correct today. Then you work it out using the stated annual change, or look up a current value for your location with a geomagnetic service.

If you measure a direction on the map and want to walk it with the compass: if magnetic north lies east of map north (east declination), you subtract the value. If it lies west (west declination), you add it. In English the memory aid is 'east is least, west is best'. For the opposite way – from compass to map – you reverse the calculation.

An invented example for illustration: you measure 100 degrees on the map, and the map states an east declination of 4 degrees. Then you set 96 degrees on the compass. With 4 degrees of west declination it would be 104 degrees. The real value for your area is on your map or from a geomagnetic service – not in this example.

If your compass has a declination adjustment, you set the value there once. The compass then corrects automatically and you no longer need to calculate on the way. Without an adjustment, write the rule on a note and attach it to the compass.

The most common mistake is not the wrong value but correcting in the wrong direction. If you add instead of subtracting, you double the error instead of removing it. So: fix one single rule, write it down, and do not work it out afresh in the field.

Just as common: correcting twice. If you have set the adjustment on the compass and then also calculate in your head, you are off again. And an adjustment screw can get knocked out of place – check it before every trip.

Whether declination matters for you depends on its size and the distance. Every angle error grows with distance: one degree gives about 17 metres of sideways offset per kilometre. If the value is small and the distance short, you can often ignore it. If it is large or the distance long, definitely correct for it.

Declination only corrects the large, uniform difference between map north and magnetic north. It does not correct local interference: iron-rich rock, nearby metal, power lines, the phone in your chest pocket. These can deflect the needle in addition.

Near the magnetic poles the compass becomes fundamentally unreliable, because the field lines there dip steeply into the ground. In most hiking areas this does not matter, but in polar regions it very much does.

Find a position you can locate on the map for certain, and a second point you can see – a church tower, a summit, a radio mast. Measure the direction on the map, convert it with your rule and then take a bearing on the point with the compass.

If both values match after correction, your rule is right. If they differ by roughly twice the declination, you corrected in the wrong direction. This little test before a trip gives a lot of confidence – especially if you later have to follow a long bearing in fog.