True North vs. Magnetic North

SkyJeff

SkyJeff

New Member
I learned today about true north vs. magnetic north from my instructor. I find it interesting, but I'd like to know more since most of what he told me went in one ear and out the other.

I know that Magnetic north is about 15deg east of True North and that it changes every 15 years or so. Can anyone explain the difference to me a little more and how do we compensate the difference in cross country flights?
 
Magnetic north is actually constantly on the move. If you look at some of the airport diagrams on DUAT, AOPA, etc. there usually is some sort of notation that shows the average amount of variance per year.

As for converting true figures into magnetic figures, as you will with a cross-country, the secret lies in the dashed magenta lines that you will see on your sectional charts. These are called isogonic lines. Isogonic lines will look like they are at a diagonal to the lines of longitude, because they connect points of equal variation (that is the amount of difference between magnetic and true). They are marked with the amount of variation and the direction. I'm in Michigan and have an isogonic line marked "4ºW" near my home airport, which means that magnetic north is 4º to the west of true north. Therefore, you must add 4º to your true course, heading, etc. to get the corresponding magnetic value. The farther east you go (talking in terms of just being inside the U.S.), the more west the variation becomes. Running north and south right almost through the eastern plain states (a little east of the Mississippi), is an Agonic line. This line is where there is 0 variation, and as you move west from there, the variation becomes an increasing east value.

I hope this was somewhat helpful. Please note that I am not a CFI and don't have much experience in explaining/teaching concepts yet, so sorry about any fuzzy areas!
cool.gif
 
Yeah, and magnetic north is somewhere up in the middle of Nunavut, Canada, and so if you fly over the north pole, your compass isn't going to do a whole lot of good...
 
Hey skyjeff. welcome to the world of the magnetic compass. as you said magnetic north is 15* east of true. thats right but as turbojet pointed out that holds true only for the isogonic line that you happen to live along. where i live manetic is 26* east of true. but thats because im up here nearer true north in alaska whereas magnetic north is over in canada. so the compass always points to magnetic north which is understandable because its the attraction of magnets that makes it work anyway. the big reason for the difference is the fact that the earth is tilted. as im sure you learned years ago the axis of the earth is actually at that tilted angle, so its not straight up and down. the lines of flux that surround any magnet run through the same point as the axis, and the magnetic compass is aligning itself with these lines of flux. therefore the geographic north pole(right at the very top of the earth) and the magnetic(where the lines of flux run to and where the axis of the earth is) are not located at the same point on the globe. well i hope this helps a little and doesnt just confuse you even more. and hopefully someone can get on and explain it better than i possibly can
spin2.gif
 
You must log in or register to reply here.
Back
Top