Because of earth’s magnetic fields, there is a difference between the magnetic north where a compass points and the north you find on a map (called declination). Declination can be up to 20 degrees E on the West Coast and up to 20 degrees W on the East Coast.
Declination is always changing due to shifts in the Earth’s magnetic poles. Declination can be found online or on a recent topographical map.
A compass with fixed declination does not allow you to adjust for declination, therefore, you must use math to get an accurate reading. Adjustable declination compasses allow you to rotate the orienting arrow independently of the compass dial but how to do this will vary based on the compass model. Some compasses come with a small tool that can help to adjust the dial while others must be manually pushed to the correct declination.
Hold compass flat in your hand with travel arrow pointing straight away from you at the direction you’re intending to go. Rotate the bezel until the magnetized needle is inside the orienting arrow. Read the bearing by looking at the index line.