>>Just varying either the x or y axis gives hugely different heading results (including at 'steep' angles Nan)
>>
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>  // Calculate compass heading
>    var compassHeading = Math.atan(Vx / Vy);
>

>
>Have you tried replacing the Math.atan()) by the Math.atan2() ?

No, but I will. My trig's too rusty to guess what the effect might be :-}

>http://www.w3schools.com/jsref/jsref_atan2.asp