How to Use a Lensatic Compass, Part I

Declination refers to the angle between magnetic north and true north. To make sure we're all on the same sheet of music, let's quickly define both of those terms as well.

Simply stated, Magnetic North is the direction that your compass will point at all times. As you rotate the compass (or yourself), the needle will always align with magnetic north, and thus you can see what direction you are actually facing by taking a reading from the compass. Magnetic north varies by location and time. That is, depending on where you are on the earth the direction that your compass indicates as north will be slightly different, potentially by as much as 30 degrees or more. Additionally, magnetic north is shown to change by several degrees in any given location over the span of 100 years. That means maps drawn up with readings from the 1800's will no longer be accurate. This is all due to changing fields around the earth, iron deposits, and local geography changing.

True north is defined as the direction of travel if you were to move directly towards the north pole. One ways of finding it without knowing magnetic north in the area is to look for Polaris in the sky, also known as the north celestial pole. This is as close to true north as you're likely to get.

Maps are drawn relative to true north, and compasses give you bearing relative to magnetic north. So if you've been following thus far, you can see the problem. If you have a map drawn to true north and a compass that is telling you north is 30 degrees to the east of that, you're actually travelling NNE relative to the map.

Cartographers reconcile this problem by giving the local declination angle when they produce maps. This means that you should look for it (called the GMA, or Grid Magnetic Angle) and adjust your compass accordingly. On a lensatic compass you can typically rotate and lock the dial such that your readings will automatically account for the declination angle, and this is the easiest and most fool proof way of addressing the issue.

Declination angles are expressed on maps in several different ways. For example, 10° W and -10° are the same way of expressing declination. Similarly, some maps will have an angle drawn on them with the declination angle indicated at the vertex of the drawing.


Anoop A said...

Thank You very much for your interesting blog. I found it extremly informative regading the lensatic compass. Hope you could post even more articles & perhaps videos on how to use the compass for effective navigation.

la strada said...

Wow! This was so interesting. I've bookmarked it to re-read later. I can already think of fun ways to use this science with my boys. We live on a lot of family land loaded with trees and different terrain. Wouldn't it be fun to set a particular spot and as a game see if they could find it? With teenagers I might have to make this little Easter egg hunt end with the finding of the queen's nest (I think that's what you call it). Translation: the spot will hold treasure in the form of money, etc.