# How to Calculate Magnetic Compass Deviation and Variation

## Magnetic Compass Deviation vs Variation – Video Transcript

Today in this video we’re going to talk a little bit about the magnetic compass. Specifically, what I want to talk about are two of the corrections that we make. One is called deviation and the other is called variation. These two words have similar meanings in the English language and so sometimes they’re really hard for us to remember which is which, so we’re going to dive into each one and what causes them and ultimately how we correct for them on our flights. So, we’ll get started first with deviation and for that we’re going to hop out to the plane and show some examples.

**Deviation**

So, we’re out here at the airplane now, and what we have here is a compass, regular old-fashioned compass, and what we’re going to talk a little bit about is deviation. Really what deviation is, is simply a magnetic interference that causes the compass to show incorrectly. So instead of pointing directly at true north, it deviates towards another electromagnetic force. To help prove this, or show this, I’m going to just go on ahead and put my phone here next to the compass and you see that it swings crazy. That’s just because my phone has electricity moving through it and ultimately also a little bit of magnetism and that just results in this magnetic force causing the compass to sway. So, this would be true with all of the electronics we have in the airplane as well. Obviously not nearly as extreme as like what my cell phone did, but still could have some effect. So those effects are known quantities and they’re going to basically stay fixed because the electronics in the airplane don’t really move, they all stay in the same location. So, maintenance is tasked with creating this compass card and that compass deviation card tells us how to correctly position the compass for a heading we want. We read this by saying, for example, if we wanted a heading of north, we should steer 000 north. So, there’s no deviation on this particular compass on a heading of north. However, if we looked at a heading of east, so if we wanted to fly a magnetic heading of East, we would actually make the compass say 086. So, there’s four degrees of deviation in that case. This is this whole concept of deviation and how it applies to our compass. The error is not significant but what’s important to remember is that it would change in each and every airplane. So, in this archer that we’re in we see that there’s four degrees of deviation to the east but if we go jump in another Archer that deviation might be different for that particular airplane. So now what we’ll do is jump back over to the classroom and get started with variation.

**Variation**

Okay so now we’re back in the classroom again. Now we’re going to talk a little bit more about variation. Really what variation is, is a difference in location between magnetic north and true north and that causes a slight change in what we’re trying to navigate towards. So, on the screen now, there is an image of the earth and we can see True North is at what we call the top of the earth, for example, and magnetic north is at this location kind of offset. In fact, magnetic north is constantly moving. It moves a little bit every year. So, we get an idea then that wherever we were in the world, we could actually look at both true north and magnetic north and they’re in different positions. So, on the board here, I have an example of how that would work in action. Let’s say we’re planning out our flight, and as we plan out our flight obviously, we use charts. Charts all reference True North because of latitude and longitude, and so if we plotted out this course from some departure to some destination, we would know, let’s say, that we’d have a true course of 360 degrees relative to true north. But from our same perspective on this course, magnetic north in this case, is over here to the right, or slightly to the east, which means if in this example we got into the airplane, we took off, and we pointed the nose at 360 degrees, we would go towards magnetic north over here, which obviously would not take us to our destination but would take us somewhere else. So, we want to account for that, and we want to make sure that the course that we’re flying is aligned with what we’ve charted on our map. So, what we do then is apply that correction value. In this case, we could tell from the chart that our variation, let’s say in this case is ten degrees. So, the angle between our direct course to magnetic north versus true north is roughly ten degrees, in this particular example. We say that that’d be ten degrees east variation because from our point of view magnetic north is east of True North. If it was the other way, then we’d call it west variation. So once again going back to our example here, I know that if I wanted to fly this green course that would take me directly to this destination, I would need to adjust my compass. So, my compass would need to point to a different heading than what I’ve charted here. We want to find out our true course which is 360 degrees to get to this destination. What does that translate to as far as our magnetic compass? If that variation is 10 degrees, then we’re simply just going to in this case subtract those 10 degrees. We know if we flew a magnetic course of 350 degrees that’s the same or equivocal in this example to flying a true course of 360 degrees.

Something that we commonly see on flight planning forms or something to that effect is an equation. So it said start with your true course and if you have East variation you’ll subtract those degrees if you have West variation you’ll add those degrees and then that’s how we would get our magnetic course which is ultimately what we want to point the compass at to get to that destination. Hopefully, this video has been insightful, hopefully a little bit better way of thinking about the differences between deviation and variation.