Magnetic Declination and Using a Compass

Hiking CompassWhat do you do when your GPS dies?  Hopefully, like the Boy Scout Motto, you are prepared for this situation with a map and compass.  One major concern with reading the compass is calculating the difference between magnetic north and true north.  Guest writer Ken Walker has created a great article discussing how to make this adjustment.

How Magnetic Declination Effects Your Heading

By Ken D Walker

When hiking in the forest, it’s a good idea to carry with you a topographical map of the area, along with a compass. A good compass (a map reading compass) will allow you to shoot bearings right on the map. That way, as long as you know your position, you can quickly calculate the direction you need to travel to reach any area on your map.

One thing that is often overlooked when reading a map like this is magnetic declination. Let’s talk about what it is first, and then how we deal with it.

The earth is not a perfect circle. It is more oblong shaped. It rotates at an axis located from the North Pole, through the Earth to the South Pole.

These locations are called TRUE NORTH and TRUE SOUTH. The Earth emits a magnetic field that merges at the North Pole, but though it would seem to make sense that this occurs at TRUE NORTH, it in fact, does not. It is a bit offset.

The problem is, when we use a compass, the needle is attracted to this MAGNETIC NORTH. Since the terrain and lines of latitude and longitude are arranged based on TRUE NORTH, we have a need to adjust for MAGNETIC NORTH. That’s where declination comes in.

Because of the way Magnetic North and True North are arranged, the declination, or difference, is different in different areas. For example, in Ohio, the difference may be 4 degrees. In Illinois, it may be 6 degrees.

For this reason, topographical maps often show declination. This is a number that needs to be added or subtracted to your reading.

If, for example, let’s say your map shows a declination of 5 degrees West. You use your compass or protractor to get a bearing from your current location to an adjacent hilltop, and get a reading of 33 degrees. Going from the map to the compass, you would need to ADD 5 degrees and set your compass accordingly (38 degrees).

Your heading of 38 degrees (by compass) would put you on track to your destination.

Ken Walker loves hiking in deep woods and navigating by map and compass. Besides map and compass, you might also want to bring one of the Bushnell spotting scopes like one of the Bushnell spotting scopes which make it easy to site terrain features at a greater distance.

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So, the next time you are on a trail (or off) and your GPS falls of the side of a cliff or the batteries go dead, your compass and map shall light the way.


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