# How can I use a magnetic declination chart to calculate true track, magnetic track and magnetic variation?

If I am given values such as 30 degrees south and 75 degrees east with a true track value such as 70 degrees, how would I calculate the magnetic track?

I understand that True track = Magnetic + Variation. But I'm not sure how to use the magnetic declination chart.

This is my attempt at such values:

• What exactly don't you understand about the chart? Nov 10, 2019 at 17:02
• I edited the post to show what I currently understand on how to tackle the issue, the x marks the 30S 75E with dots going roughly 70 degrees on either side. Nov 10, 2019 at 17:14

The chart shows you lines of equal declination $$D$$. You identify your location on the chart (which you have done and marked with an X in your second image). Then you determine on which line you are. It looks like the point is between the -26° line and the -28° line, so let's call it $$D=-27^\circ$$. That gives you a magnetic track of:

$$\varphi_\text{mag} = \varphi_\text{true} - D = 70^\circ - (-27^\circ) = 97^\circ$$

• Thank you, that makes perfect sense. Nov 10, 2019 at 17:30
• Oh i see, I thought it was the other way around for some reason. My professor gave me that equation so I'll have to question him about it. Thanks for clarifying. Nov 10, 2019 at 17:40
• @SillySquishy Actually, you were right, I mixed up North position with the track direction. My bad :( Nov 10, 2019 at 17:42
• Oh haha no worries. Thanks again for the help :) Nov 10, 2019 at 17:49

We've had a question like this before. I don't recall the title exactly. The calculation is only for the exact spot where you are located right now (or at some hypothetical point in the flight.) It has nothing to do with drawing long lines all the way across the map. If your track crosses multiple lines of declination then you'll be repeating the calculation at multiple different points during the course of the flight or during the course of the flight planning. But you were asking for one specific point so there is no need to draw a line or think about more than one point. (A line is a collection of an infinite number of points.)

Why are you drawing a track all the way across the map from 70 N to 70 S? And why did you draw it along a track line of around 150 true (or 330 true if you are going the other way?)

That would be an interesting flight to make but it is not related to the question you seem to be asking.

• I made the track all the way across the map because I didn't know how else to incorporate the true track of 70 degrees into my calculation Nov 10, 2019 at 17:25
• Yes i drew it incorrectly, my bad. Thanks for the help, I understand it a lot better now. Nov 10, 2019 at 17:30