Map Navigation 101: Declination

map and compass

Map reading is a critical skill for adventurers.  It may seem like your Geographical Positioning System unit is a wonderful thing – and it is – until it dies without warning.  When that happens, you must have a map and compass with you and know how to use them.  I’ve been thinking about putting together a series of instructional videos that demonstrate how to navigate with a map and compass.  Sierra Designs and Andrew Skurka  beat me to it with better production values than I would have presented.  First,  I’ll provide a little background to help you better follow the video, then I’ll present the video.

Choosing Your Compass

Andrew displays two compasses in the video.  The better compass looks like the Suunto M-3 compass.  It costs a little more, but has the advantages of luminescent markings, a built-in magnifying lens and adjustable declination correction.   It’s a solid, reliable compass in almost any situation you might get into.

The other compass looks like an inexpensive Silva compass similar to the Silva Starter Compass. It’s also a very nice compass, but for the lower price it lacks the durability and the adjustable declination correction feature of the Suunto.

With the exception of the special adjustment that Andrew shows in the video, both compasses work about the same way.  Personally I use the less expensive Silva Starter compass, but only when necessary.  For the most part I rely on my GPS equipment, but I always carry the compass in my emergency kit.

What is Declination?

In this first video Andrew explains declination.  Declination is the difference between true north, that you want at the top of your map, and magnetic north that is at an angle to your map.  Your compass points toward magnetic north, so its important to know the difference between the two.  You can use the north star at night to pinpoint true north, but during the day you must make an adjustment to your compass to identify true north.

declinationFortunately the maps you should be using in the wilderness, like USGS maps, include a little diagram and a notation that tells you this difference as shown in the photo to the left.  The star at the top represents the north star and true north. ‘MN’ means magnetic north, the direction our compass points.  In this example, from the Yamsay Mountain, Oregon, USGS quad, magnetic north is 15 degrees and 20 minutes east of true north. As Andrew illustrates in the video, that’s because magnetic north is south of the north pole in Canada.  Thus locations, roughly, west of the Mississippi River will find their compass pointing east of true north while those east of the Mississippi River will find their compass pointing west of true north.

Orienting Your Map

To get the top of your map pointing at true north you can do one of two things:

  1. point the top of the map toward magnetic north using your compass, then rotate the map clockwise or counterclockwise, as indicated in the diagram on the map, the number of degrees noted on the map.
  2. set the rotating dial on your compass so that when the needle points at magnetic north, the top of the compass points toward true north, using the degree notation on the map.  You just need to rotate the dial the degrees indicated on the map in the proper direction and your compass will be properly set. Thus, the arrow, when pointing toward magnetic north, will fit inside the orienting arrow engraved on the bottom of the rotating dial.

The second option is generally preferred because once you have your compass set, you don’t need to be rotating the appropriate number of degrees each time you look at the map.  Instead you just hold your compass so the long edge is against the side of the map and parallel to it and move until the north arrow is inside the arrow outline on the inside of your compass.

Once your map is oriented properly, the landmarks, trails and other things on the map will be properly aligned with those items in the real world.  This makes interpreting the map and understanding what you are seeing around you much easier.

The Video

 

 

Please Share

Leave a Reply

Your email address will not be published. Required fields are marked *

Pin It on Pinterest