Compassing 101
Declination
So far, all bearings we've used are based on magnetic north. Normally, that's all a letterboxer will deal with. But on rare occasions, someone likes to trick you and give bearings based on true north. It would be nice if magnetic north was the same as true north, but it isn't. A compass will point to the magnetic North Pole, located about 1,400 miles south of the true North Pole, off the northern coast of Canada near Bathurst Island. And that means a headache for you.
The difference in degrees between the direction the compass needle points and the direction of true north is called the declination. It would be nice if the declination was the same everywhere and you could convert from one to the other with a formula—like you can do with Celsius and Fahrenheit—but it doesn't work that way. Declination varies dramatically all over the planet—as much as 50 degrees just within the United States—and the only way to find the correct declination for a given area is by looking it up. The few letterbox clues that use degrees based on true north usually include the declination to make things easier for you. (I never understood that, since, presumably, if they want to make things easier for you, they wouldn't use true north for their measurements in the first place.)
There are several sources one can use to find the declination for a given area. A good topographic map will show the declination for the area it covers, as will many trail or nature guides. You could also try Internet searches looking for websites that show the declination for your area. Search using "magnetic declination city" where you replace 'city' with the nearest city (or whatever point of reference is most convenient) for the area whose declination you want. The closer and smaller your point of reference, the more accurate the declination will be.
Okay, you've finally found the declination for your area. How do you use this number to convert from degrees based on true north to degrees based on magnetic north? If your declination is west, add the declination to the degree that must be converted.
Let's use an example to make it clear. Let's say you are in an area where the declination is 5°W. You want to walk at 185° based on true north. Since the magnetic declination is west, you add 5 degrees to 185 and get 190. And by using a bearing of 190° on your compass, you'll be set!
If the magnetic declination is east, you'll subtract the degrees of declination from the true north measurement to get magnetic north. Another example: Let's say you are in an area where the declination is 12°E. You want to follow 305° from true north. Since the declination is east, you subtract 12 from 305 and get 293°. Set your compass to that bearing and your off!
What if you're creating a clue and want to make it difficult for others by giving clues using degrees based on true north? You'll do the opposite as before: If the declination is west, subtract the declination from magnetic north, and if the declination is east, add the declination to the magnetic north reading.
If this all sounds confusing, it's because it is. Which is also why it's rarely used in letterboxing clues—magnetic north works just fine unless you're comparing a map (based on true north) to a compass reading (based on magnetic north). Since letterboxers rarely use both the map and the compass at the same time, there's rarely a need to convert between the two—unless, of course, the person writing the clue wants to make finders of their box jump through a few extra hoops.
To summarize the rules for converting between true north and magnetic north, you can use the table below:
True north -> magnetic north
Declination west: Add
Declination east: SubtractMagnetic north -> True north
Declination west: Subtract
Declination east: Add