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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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  1. Conant
    6 March 2011 at 11:53

    This method is reliable only to within about two degrees, as the axis of the Earth’s rotation doesn’t point in exactly in the right place all of the time, in part because of the orbit of the Earth around the sun. The same problem arises in determining your latitude by measuring the angle between the horizon and the North Star. The Nautical Almanac has a table of corrections that can be used.

    See http://books.google.com/books?id=duzio8OkjPoC&pg=PA355&lpg=PA355&dq=polaris+correction+for+true+north+or+latitude&source=bl&ots=eupmZiHH9V&sig=SGSX1y_GWR58U6rLvvZ5TpnjObk&hl=en&ei=X7lzTa62JoLGgAeXirVV&sa=X&oi=book_result&ct=result&resnum=5&ved=0CDQQ6AEwBA#v=onepage&q=polaris%20correction%20for%20true%20north%20or%20latitude&f=false

  2. Rusty W
    18 March 2011 at 21:50

    There’s something to be said for the art of “dead-reckoning.” I was out training with my local militia unit about a year ago, when someone came up to me and asked me what direction was north. I know it was about 11am, so I looked up at the sun, then quickly pointed in a certain direction (without looking at a compass first). The questioner said he thought north was about 90 degrees different from where I was pointing. I held my arm pointing in the same direction, then told him to get out his compass. Damned if I wasn’t within 5-10 degrees of magnetic north! Of course, that’s a skill I learned way back in the Boy Scouts. You also have to adjust for the month of the year, as in the winter the sun never goes from east to west, but actually from southeast to southwest (Northern Hemisphere).

    That also works if you can find the North Star at night (Northern Hemisphere, of course). Of course, if it’s totally overcast day or night then it won’t work.

    Rusty W

  1. 29 April 2011 at 14:15

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