Declination: Why Your Compass Doesn’t Point North

Your compass don’t point true north! Sounds a little like a redneck insult, doesn’t it? Well, it’s just flat truth (with bad grammar). Try compass navigation on the assumption that the little needle’s pointing straight north and you are going to be more or less led astray. How far astray depends on the magnetic declination in your region.

On this diagram is at true north. This shows an eastward magnetic declination of about +30 degrees, because the compass needle points 30 degrees east of true north.*

The magnetic pole of the planet doesn’t run right through the geographical center in the earth and terminate right where the spinning axis emerges from a globe. Why not? Flows of magnetic iron under the skin of the planet, mostly. Since those flows change over time, so does the position of magnetic north.

In some areas, deposits of magnetic ores in the rock layers also pulls compass needles astray. That means the magnetic declination varies in different places, as well as varying over time.

Here’s an overview of showing how declinations change across the U.S. Here in North Missouri, we’re near zero; but that’s not true everywhere.

How can you find the magnetic declination for an area?

Fortunately, this is easy. And while declination does change over time, it doesn’t change much over days or months. You might not want to use a decades-old map, but information from a few years back is still close.

One way to find the current declination is to click on this handy link to magnetic-declination.com and tell it where you want to know about. You can also read the local declination off of a decent topographical map.

If you’re looking at a free (ok, paid for already with tax dollars) USGS topographical map, there will be a diagram at the bottom left. The line with the star is true north. The line labeled MN is magnetic north.

Here’s how a topo map shows true north (may be shown as a star) and magnetic north. Grid north? That’s a subject for another post. Also note the date of the map and the description of how declination will change over time.

What’s up with the positive/negative, east/west declinations?

When magnetic north is east of true north, it’s shown as a positive number. Western magnetic norths are shown as negatives. The image just above showed a positive (easterly) magnetic north.

How do you use the declination?

There are compasses that let you take declination into account when navigating with a map and known declination. They’re great, and I’ll talk about them in a later post. For today, though, let’s look at what you do if you’ve just got a plain jane, “needle and degree readings only” style compass.

Declination when you have no map 

Say it’s late in the afternoon and you know you have to keep walking through the night. You use your compass and find the heading to some distant point you can still see is 90 degrees. (So you’re heading eastward, but maybe a little northeast or southeast depending on declination.) Your mission is to follow this bearing until you hole up for the night.

Congratulations! You can safely ignore declination in this setting, since its the same when you were deciding direction as when following the direction. Just keep heading to 90 degrees.

Declination when you read the heading off the map and use the compass to go there

Your map says the destination you want is due east of where you are now, so it’s at 90 degrees. You’ve already discovered the declination in this area is 10 degrees west. That means a declination of -10, since eastern values are positive and western values negative.

1) Start with the map heading: 90 degrees.

2) Subtract the local declination from the map heading: 90 – (-10), which means 90 + 10, so 100 degrees is your corrected heading.

3) Use your compass to walk toward 100 degrees.

Declination when you have a heading from a compass and want to use a map to see what’s over there.

You see a peak – or a skyscraper – to the east. You want to find out where you are relative to this landmark on your map. You’re still in an area with a declination of 10 degrees west (-10). Here’s the drill:

1) Using your compass, you determine the heading to the tall thing is 90 degrees.

2) Add the local declination to the compass reading. 90 + (-10) = 90 – 10 = 80.

3) The true heading is 80 degrees. Find the tall thingy on the map; you are somewhere along the line that produces a heading of 80 degrees to the tall thing.

Bonus round! Do the same process with a second landmark. On your map, draw a bearing line that has the first tall thing at a heading of 80 degrees. Draw another line that represents the second bearing to the second landmark. You’re about where those two lines cross on your map.

This is the first of a planned series on compass navigation. I’ve done a little of that in the field (and under the waves) and not gotten lost. The experience both taught me the value of the skills and convinced me that I needed to up my game. I’m on a self-study course at the moment and mean to write up pieces of it as I go to help others develop the skills too. I have a feeling that if the GPS system fails, there are going to be a lot of Seriously lost people. Let’s not be among them.

*Thanks for the image to odder [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons

Salty’s Note: Sorry if you have seen this one from Last Week (i,e. if you are a subscriber) the post had some coding difficulties and we had to pull it. We replaced it with another post from our que so you you didn’t miss out. There’s something about the wording n this post that is driving our wordpress site nuts, not a clue what it is, so sorry that it’s a bit ugly.