Making Sense of Declination

Declination is a term that can frustrate the hiker. Does the navigator add or subtract the declination?

I’ll offer a few suggestions to keep the navigator on track.

First, let’s discuss some terms relating to a compass. Degrees true refer to an angular measurement from true north; the North Pole. Degrees magnetic refers to the angular measurement from magnetic north. Magnetic north and true north in most cases are not the same. So, if a hiker travels to the east or 090° magnetic that direction is equivalent to 106° degrees true. The difference is the declination of an area.

Generally speaking, people hiking on in the Pacific Northwest work with Easterly declination while those on the Atlantic coast work with Westerly declination.

Topographic maps identify the local declination in the small declination diagram found at the bottom of the map.

That said, in many cases the map’s declination data may very well be out of date as declination changes with time. To get the most current information I use the site www.magnetic-declination.com . At that site I’ll provide the rough coordinates of the area that I’ll be hiking through to obtain the current declination.

Now, to make this information of any use I’ll need to apply this to my map and compass.

For example, I am going to plan a hike from a local trail head to Scout Lake. I will use my magnetic compass like a protractor and determine that the direction is roughly 150° (degrees true.) (Maps direction is almost always provided in degrees true.) While measuring the bearing I take no notice of the magnetic needle’s movement. I am purely using the baseplate and rotating housing like a protractor.  (See the compass components below.)

I need to convert the true bearing to magnetic so that I can use my baseplate compass. It’s time to do a bit of math to determine the magnetic compass heading. The known values are:

1.  A declination of 16°East

1. A true direction of travel of 150°

1. Because of the easterly declination, solving for the correct direction requires me to subtract the declination (16°E) from the true bearing (150°). The magnetic compass heading is 134° magnetic.

1. To travel to Scout Lake, the rotating dial is turned such that 134° is aligned with the direction of travel arrow. I’ll rotate my body to align the red magnetic needle directly above the baseplate orienting arrow. I will now follow the direction provided by the direction of travel arrow.

In my college navigation class I have found that drawing a diagram really helps my students to understand the math. The diagram below visually represents the math outlined in a – c (above.)  In the diagram below the dashed blue arc represents the unknown value for the magnetic heading. Because the true direction value to Scout Lake is known, subtracting declination provides the needed information.

For more about declination read my post on declination and June Fleming’s book Staying Found.