Magnetic vs. geographic north
A compass points to the magnetic north pole — a point in the Canadian Arctic that slowly shifts due to changes in the Earth's core. Geographic or 'true' north is the rotation axis of the Earth. The difference between the two directions is called magnetic declination.
Why this matters
In the Netherlands, magnetic declination is currently about 1–3° east. That sounds small, but over a distance of 10 kilometres, a 2° error results in a deviation of 350 metres. In mountain terrain or on longer marches, this can be the difference between reaching the intended objective or missing it entirely.
Variation by location
Declination varies greatly by location on Earth. In Norway it can be 4–8° east; in eastern Canada up to 20° west. In some areas, such as Alaska and parts of Russia, declination is so large that ignoring it causes navigation errors of kilometres.
Change over time
The magnetic north pole moves at an average of 55 km per year (even faster in recent decades). As a result, declination slowly changes over time. A map from 10 years ago may contain an outdated declination. Always check the current declination via the MilMap declination calculator or the NOAA database.
Correcting for declination
To compensate from magnetic north to geographic north: with east declination subtract the declination angle from the compass bearing. With west declination add the declination angle. Memory aid: "East is least, West is best."
Using the MilMap declination calculator
MilMap has a built-in declination calculator that computes the current magnetic declination for any location on Earth, based on the latest World Magnetic Model (WMM) data from NOAA. Enter coordinates or a place name and you get the exact, current declination including the annual rate of change.