When navigating with a map and a compass, it is important to understand that the true or geographical north (North Pole) is not the same as magnetic north. The angle between true north and magnetic north is called declination and is either east or west of magnetic north depending on where you are on Earth.
East declination means that locally the Earth’s magnetic field points in a direction that will pass the North Pole on the east side. West declination passes on the west side. In many parts of the world declination is so small that you do not need to take it into consideration in practical outdoor navigation. However, in some places around the declination is significant and must be accounted for at all times in order to navigate accurately.
You should always check your local declination from a trusted source before you start navigating. The declination angle changes over time due to the fluctuation of Earth’s magnetic field.
Many Suunto compasses provide either fixed or adjustable declination correction. The fixed declination correction is an additional scale under the compass needle. This is used by first taking the direction with the protractor and then turning until the compass needle points at the declination scales according to the local declination. This must be done every time the compass is read.
Adjustable declination correction is set once when starting to navigate by turning the North arrow in the bottom of the capsule to the angle that corresponds with your local declination. The compasses are equipped with a small screwdriver and a printed declination scale to assist in making this adjustment.
After setting the declination correction, the compass can be used without having to account for the declination until you move to a new location with a different declination.
There is a third North called map north. The map may show a difference in the direction towards true north depending on if you are on the middle meridian of the map or at the east or west edge of the map. If this is the case, the printed map will have a description of this perspective error together with the declination information. In outdoor navigation, landmarks are usually appearing so often that this effect can be ignored.
In addition to declination, other phenomena can affect the direction a compass points. The ground may contain magnetic minerals, like magnetite, that can have a big impact on the compass. It is important to regularly check your progress on the map while moving to be able to correct for variations causes by such minerals.
Power lines and other human constructions may also impact a compass reading. Crossing bridges or underground pipes, even working on a surface with nails can cause temporary changes in the compass reading. Magnetic materials or items you are carrying can affect your compass reading as well.
Always be aware of what is with you and around when using a compass. Distance yourself from possible disruptions to get a good reading.