Compassing 101
Declination
The declination is the angle between
true north and magnetic north.
In this case, declination is negative
since magnetic north is west of true north.
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 angle between true north and the direction the compass needle points 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.
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. They do go out of date, however—magnetic declination is an annoying moving target! It's not a fast moving target, but if your topo map or guidebook is a couple of decades old, the declination could have changed significantly. Try using the Magnetic Declination for up-to-date declinations.
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? Add the declination to true north. Let's use an example to make it clear. Let's say you are in an area where the declination is 5°W (i.e. -5°). You want to walk at 185° based on true north, so add -5 to get a magnetic bearing of 180°.
Another example: Let's say you are in an area where the declination is 12°E (i.e. +12°). You want to follow 305° from true north. Set your compass to 317° (305 + 12) 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: Subtract the declination from magnetic north.