>>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


Update 2

This is true for a target in close range, where you can approx earth as a plane. I f you go farther it becomes a bit more tricky. Do you like to point a straight line (digging), great circle or what?