Map and Compass

Maps, Compasses, and Declination

Scouts will be interested to learn that the north indicated by their compasses is not really true north. Instead it points to the magnetic north pole, which is not fixed. The difference between true north and magnetic north is called declination.

This is a brief introduction to the subject. For more information, see the Boy Scout Handbook or and Orienteering merit badge pamphlet.

Use this information with Boy Scout Second Class requirement 1: Demonstrate how a compass works and how to orient a map. Explain what map symbols mean.

Magnetic Declination

What Is Magnetic Declination?

Magnetic declination is the angle between magnetic north (the direction the north end of a compass needle points) and true north. The declination changes in both space and time. So the declination for New York will be different than the declination for California. And the declination for 2010 will be different than the declination for 2013.

What Causes Magnetic Declination?

True north (which is usually the north indicated on maps) is the direction along the earth’s surface towards the geographic North Pole. Magnetic north is the direction of the magnetic North Pole. The magnetic north pole is not fixed, but moves over time due to changes in the Earth’s magnetic core.

How Is Declination Shown on Maps?

There are several ways declination might be indicated on a map:

  • There might be an indication of the direction of magnetic north or MN on the map.
  • The map might indicate the declination as an angle. For example, 7W would indicate that magnetic north is 7 degrees counter-clockwise from true north
  • It might also be indicated as a signed number. For example -7 would be the same as 7W above, but 7 would indicate that the direction is 7 degrees clockwise instead.

Remember, that the magnetic declination changes over time, so the declination stated on maps will not be accurate if they are several years old.

How Is Declination Used with a Compass?

If your compass has an adjustable baseplate, you can adjust the red orienting arrow on the baseplate to compensate for declination. If the declination is 7E, move the baseplate’s orienting arrow 7° W of north to compensate.

If your compass does not have an adjustable baseplate, you will need to add or subtract the declination to the bearing. For example, if the reading is 250 degrees and the declination is 7E, you are actually facing 257 degrees. So, to actually head 250 degrees, you need to get a reading of 243 degrees.

You can also use a ruler to draw lines for magnetic north on your map. You then use these lines and your compass to orient the map correctly.

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2 Responses to Maps, Compasses, and Declination

  1. John Morgan October 18, 2012 at 10:52 AM #

    The difference between true north and magnetic north is called Variation. Declination is the angular height of a star above the celestial equator (analogous to latitude on the earth’s surface).

    • Matt Kelly July 2, 2013 at 6:10 PM #

      In the military, we called it ‘deviation’. Indeed, declination and right ascencion are used to find the celestial position of stars.

      Variation was a number that had to be empirically tested and it was the amount of error contributed by the metal around the compass.

      We used the pneumonic “Can Dead Men Vote Twice, At Elections?”

      Compass +/- Deviation +/- Variation = Magnetic, (Add East)

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