Tuesday, August 27, 2013

The declination cheat

I was teaching a map and compass class on Saturday for 10 highly motivated people. It was a lot of fun, and always nice to be outside teaching, particularly teaching people who are excited to get active in the outside world.

So, I was talking about Declination. The difference between the two 'north poles'. This is how I describe it:

There are two north poles, One, where Santa Claus lives, is at the very top of the planet. Santa Claus' north pole is where the vertical lines on a map run. But then there is Magnetic north. This is where your compass points. So not only is there a difference between these two places, but that difference changes.

I don't know how long I have been teaching map and compass - probably sliding up on ten years. I teach it to full classes three or four times a year, which depending on class size is 10 to 30 people, so lets say 60 people a year times 8 years is almost 500 people. Something happened for the first time on Saturday.

As I was explaining the above concept A student asked "why don't they just change the way they draw North on a map?" This is a big concept, and I think in my shock that someone figured it out before I told them prevented me from congratulating this student on being so sharp. In 500 students no one had ever just come up with this in class. Because that really is the answer. But first, you have to know how to do it the right way, which while pretty simple, is generally where people make mistakes with map & compass.

This is how you are taught to do it. You plot a course on a map using your base plate compass. You draw a nice line on the map and label it with the heading you will want to follow. Say 180º. Then, you put down the map, and grab your compass. Turn the bezel to 180º at the bearing index and put red fred in the shed (turn your body in a circle until the red end of the needle (red fred) is inside the arrow inside the bezel (the shed)). Now you are facing 180º, the direction you want to go, except really you aren't. Because declination has the needle pointing a direction you don't want to go. If the declination is 10º west You would add 10 to the bearing (because East is Least (subtract the declination) and West is Best (add the declination)) So turn your bezel to 190º put red fred in the shed and you are ready to walk that bearing.

Normally the math isn't so perfect. Normally you will want to go 137º and your declination is 9º and someone will add it when they should subtract it, and it becomes a mess. So there are two things you can do. The first is buy a compass with a settable declination. My Suunto M3d has a tiny screw driver so I can turn the inside of the bezel however many degrees the declination is. Then when I put Red fred in the shed the math is already done. But this means if I travel I have to remember to set the declination on the compass. But here is the cheat. This will save you a metric ton of worry, but I generally advise people to get good at declination first. Then employ this cheat.

On the bottom of your map there should be a diagram illustrating declination. It should look like this. Charts (and National Geographic Trails illustrated topo maps) use a compass rose, with two circles. The inside circle shows MN, and the outer circle shows geographic north. 

So this symbol is telling you that magnetic north is 13.5 degrees east. It is the angle between the star and MN. Before your trip lay out your map, and put a ruler - or better, a yard stick if you have one - on the MN line, and draw a line across your map in pencil. Then draw parallel lines all across your map. You have now 'redrawn' north on your map to match the declination. Instead of aligning your compass with the straight vertical lines of the geographic north pole you can now align it with magnetic north, where your compass is already pointing. That is your cheat. No more math. 

But the answer to my student who figured this out on his own, as to why they don't just print this on maps is this. The reason is declination is always changing, and so this would mean yearly reprinting of maps. Maybe next time I will tell you about my favorite navigational tool.... A piece of string! 


  1. Concise and helpful! This is one of those aspects of m&c navigation that gets under my skin. Never can quite keep it clear in my head.

  2. Glad it worked for you Corey. It is one of those things that can really cause trouble, and it is really pretty simple. The kicker is there are so many ways to make a simple mistake that can really bite you. For what it is worth, every time I teach a M&C class I run through declination on my own. SO if it can get under my skin, there is no shame in the fact that it gets under yours.

    thanks for stopping by.


  3. To understand declination you must first realize that there are two North Poles. We typically say that a compass points to Magnetic North, not True North. Technically, that is not exactly true. Thanks for sharing this great info; it has really been helpful with navigation. I also learnt a few tips on how to use a compass here: