True north, or geographic north, differs from magnetic north, or the north on a compass, due to the influence of the earth's magnetic field. To calculate true north, you must set the magnetic declination on your compass so that your compass now lines up with true north. If you forget to do this before setting out on a hike or geocaching expedition in which you intend to follow your compass, you'll end up off-course.
Find the magnetic declination of your location by consulting a topographical map of the area or a chart online. On a map, find the illustration of the map's scale, usually located in the lower right or left-hand corner of the map. In this same box should be a notation of the area's magnetic declination.
Set the magnetic declination on your compass. Rotate the outer ring of the compass until the indicator pin aligns with the number equal to the magnetic declination. If your compass has a declination screw instead of a ring, turn this screw until the indicator pin lines up with the declination number.
Rotate the compass until the needle points to N. This is true north.
You can also determine magnetic declination using a calculator, such as the one on the Natural Resources Canada website, if you know the latitude and longitude of your location.