Magnetic Declination: Why Compass North Differs from Map North
A compass needle points to magnetic north, while most maps are drawn to true north. The angle between them is called declination. Ignoring it can put you well off course over a long distance, so it is worth understanding before relying on a compass.
What declination is
Declination is the angle between true north and magnetic north at your location. It can be east or west and ranges from almost nothing to many degrees depending on where you are. It also changes slowly over the years as Earth's magnetic field shifts.
Check the value printed on your map, and confirm current values with an official online calculator, especially if the map is old.
How much it matters
A small angle grows into a large error with distance. Being off by a few degrees over several kilometers can mean missing a trail junction or a narrow pass entirely.
Over short distances in open terrain with clear landmarks, the effect is smaller, but it still adds up across a day.
Correcting for it
Many baseplate compasses have an adjustable declination setting, which is the simplest option: set it once for the area and read bearings directly. Otherwise, add or subtract the declination when converting between map and compass bearings, following the rule for east or west declination.
Write the correction on your map margin before the trip so you do not have to work it out in bad weather.