Magnetic North vs True North
Once you've set your bearing, you're on the right track to finding your way. But there's still another wrinkle. Magnetic north isn't the same as true north -- it's close, but no cigar. Magnetic north is always moving, and we call this margin of error declination. Declination is an angle that measures the difference between true north and magnetic north. The angle varies depending on where you are on the planet. This is why it's important to always use a current map when you're in unfamiliar territory, especially when you're trekking long distances. With short distances, the declination may only be 100 feet (30 meters) or so. But when you're trekking long distances, the margin of error could be several miles (or kilometers). Your map will tell you the declination. When you make your navigation calculations, you add or subtract that angle from the compass bearing numbers. Some compasses only require you to make that adjustment once for your entire trip -- check your compass instructions for more about setting the declination.
Again, learning to read a compass and topographical map requires preparation and skill. Read the instructions that came with your compass, check your library, outdoor gear store or search for a tutorial online. Try doing a practice run at home before you go out in unfamiliar backcountry.
If using a compass requires so much calculation and skill, would a GPS make things any easier? Find out on the next page.