Consult a map and determine your location. Locate the nearest horizontal line crossing the map to determine your approximate latitude (for example, 35 degrees north latitude). Locate the nearest vertical line crossing the map to determine your approximate longitude (for example, 90 degrees west longitude).
Consult a magnetic declination chart and match your location to the closest declination line; this value shows the difference between magnetic north and geographic north at your location.
Hold the compass steady and turn so the compass needle points at the value of declination (for example, if your declination is 15 degrees west of geographic north, turn 15 degrees to the west so the needle is pointing to 345 degrees on a 360-degree dial). At this point the needle is pointing to true north, not magnetic north.
Set your compass to record that the current direction is true north by either turning the compass dial so its "north" matches the needle, or by making a small mark with waterproof paint where the needle is pointing.
Consider this setting to be north in all future navigation, as the compass needle will now point to true north instead of magnetic north, greatly reducing inaccuracies when comparing compass readings to maps.
