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I want to find the magnetic heading (from north) from a point $A$ to a point $B$ on a $\triangle ABC$. $A, B and C$ are moving and so the angles can be any value. as an example, the triangle could be this shape.

Lengths $a, b$ and $c$ are known and therefore using the law of cosines the angles at A, B and C can be found. Is there a way to use these values to find the magnetic heading or is it completely removed from these angles?

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No, the heading cannot be found from the information provided. Think of it this way: The triangle in your example could be rotated by any amount from 0 to 2Pi without changing the side lengths or angles. –  Pieter Geerkens Apr 22 '13 at 22:20
    
If you know the $(x,y)$ coordinates of each point, then you can find it –  Quincunx Apr 22 '13 at 22:25

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If the lengths are know, the angles of the triangle are known as well. But you have not described any information that would allow determining the rotation of the triangle in the plane. Imagine rotating the triangle around A by 15 degrees clockwise-that will change the compass heading but nothing in your data.

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That's what I thought :/ Think I'm going to have to write something to use the heading the points move at (they are bots with compasses) and then check how the angles change as they move and create a heading from that. –  mark mcmurray Apr 22 '13 at 22:45

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