>>>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