>>>>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?
>
>Hi,
>Given two locations Android's BearingTo() function already returns the bearing as the bearing based on shortest path using WGS84.
>The solution turned out to be simple - see my reply to Al

I can not realy follow it.

As long as this is the great circle it's the shortest distance. If it's just heading - this will not work

But to test is simple
Select a destination same latitude as you are but 90° east. If your calculation ends up just east it's not the shortest path. Should more north in it.