>On an Android device you can obtain:
>
>(a) From the magnetometer: The device orientation (i.e orientation relative to N).
>(b) From the GPS : Bearing from current location to a specified other location.
>
>Both of the values are returned as the number of degrees E. of North.
>So they are in the range of +0- +180 or -1 to -179 (-180 might be possible - haven't checked)
>
>I want to put an arrow on the screen pointing from the current location to the specified location (in the range of 0-360)
>What is the most efficient algorithm ?
>
>Actually the magnetometer reading if for Magnetic N. and the bearing is True North but I can adjust for that later (same for the device window orientation)
>
>I can see a few implementations on the net but I've a feeling that they could be simplified..........
>
>Suggestions ?

son in law, not me:

angle_of_arrow = atan2(target_y-ownpos_y,target_x-ownpos_x)-atan2(Magnetothing_y,Magnetothing_x) (+90°*n?)

last term is possible offset.

update:

atan2(Magnetothing_y,Magnetothing_x) returns an angle. Skip it
Just norm to angle of the magnetometer to 0-360 by adding 180.

possibly you need to play with sign of the number but in general