If your compass does not have a magnetic declination adjustment, you may need to add or subtract degrees from your direction of travel to compensate for the difference between true north and magnetic north in your area of travel. I say may, because if you are using a compass to navigate to and from a destination without a map, there is no pressing need to account for the difference between true north (which your map is oriented to) and magnetic north. The exception is when you are also using sun angles and other techniques to determine direction and you are in an area with a huge difference between magnetic and true north. Here, making a correction would help you switch back and forth between compass and sun/celestial navigation.
If you are using a map (which you should be) you will first need to determine the magnetic declination for your area (see Finding Declination for Your Area). If it is an east declination you will subtract the appropriate number of degrees when taking bearings from a map (east is least) and add degrees when you sight an object with your compass and want to transfer that bearing to the map.
For example, If you are going for a cross-country hike near Portland, Oregon where you have an approximate magnetic declination of 20 degrees east (this means your compass is pointing toward the magnetic north pole which is 20 degrees east of north on the map). Let’s say you place your compass on the map, lining up the edge of the compass with your starting point and your destination (or landmark of interest). To determine your bearing, you twist the dial until the orienting lines on the bottom of the compass line up with north on the map, then read the compass direction. Let’s say this direction, which is true north, is 85 degrees. You will now need to twist the dial to set your compass to 65 degrees before using your compass to navigate toward that destination.
Let’s do another example. If you are hiking in upstate New York, the magnetic declination is closer to 15 degrees west. If you were to plot a course of 50 degrees on a map, you would add 15 degrees and set your compass to 65 degrees for navigation.
There are several mnemonic phrases and rhymes to help you remember how to make these corrections. I created my own to reflect my forestry profession:
“Every Day Tree Tops Make Shadows”
The translation is, “East Declination True to Magnetic Subtract.” In other words, if I am in an east declination area and plotting a course on a map (true north) and need to correct it to magnetic north (compass) I subtract the appropriate number of degrees when I set the compass. Conversely, I add degrees if I am converting between a compass bearing (magnetic) and map bearing (true) in this same east declination setting. Adding and subtracting is reversed in a west declination area.