Land navigation skills: magnetic declination
Go out some clear night with your compass. Look up and find the North Star. Draw an imaginary line from the North Star to the horizon, intersecting it at a right angle. Make note of some landmark at that point. Now take your compass and establish a bearing to 0 degrees, noting some landmark at that point. You’ll note, if you’re careful, that the landmark you noted from the position of Polaris is offset from the other landmark by a small amount, usually five to ten degrees. The difference between the two is called magnetic declination. It occurs because of a number of factors, including the location and movement of magnetic material in Earth’s crust and core.
If the magnetic north is east of the geographic north, then the declination is positive; if it is west of geographic north, it is negative. The reason we care about declination is that while our maps are drawn with reference to true north, our compasses always point to magnetic north. When using them together, we just need to add or subtract the declination to get the true bearing.
If your declination is 5 degrees west, and your travel bearing is 275 degrees true, then add five degrees from your magnetic bearing to get the proper reading for the compass (280 degrees).
If your declination is 5 degrees east, and your travel bearing is 204 degrees true, then subtract five degrees from your magnetic bearing to get the proper reading for the compass (199 degrees).
Finally, NOAA’s National Geophysical Data Center gathers data about magnetic declination and maintains a database here. Get in the habit of checking your maps against their data periodically, and updating as necessary.
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