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Friday, April 2, 2010

Hiking Tips: Understanding Declination for Navigating

When you navigate the old-fashioned way, with the use of a map and a compass, it’s important to know where north is. After all, on your topo map is a fancy arrow pointing straight to the top of the map and indicating north. So, since your topo map was drawn to represent the terrain where you are hiking, you should be able to easily determine where north is with regard to where you are standing, right? Just look around at the terrain, compare it to the topographic markings on your map and follow the arrow to north. Dead easy, right?  Hmmm. Maybe not so easy.

Hiking compass
Image via Wikipedia
Oh, wait. The compass. Even easier. How did I forget? The needle points to north.
The question is, “Which north?” You mean to say there is more than one north?
I’m afraid so. There’s true north and magnetic north. And they are not the same. The problem is that your map is oriented to true north and your compass will consistently point to magnetic north. Rarely will the two be the same.
The angular difference between the two norths is called “declination”.
So, which north is best for navigating with a map and compass? Well, you’ll have to rely on your compass which will always point to magnetic north and then either add or subtract the declination to find true north.
On the bottom margin of your topographic map you’ll find the declination angle for the area covered by the map. It will tell you whether to add or subtract a few degrees to determine true north.
On many good compasses, you can preset this declination and then read your compass straight from the needle. It’s a good idea to set your compass before you take off on your hike. Then you won’t have to worry about it while hiking and navigating.
By Richard Davidian, Ph.D.
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