>>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 ?
>
>I'd think it would just be addition/subtraction:
>
>1. Convert both GPS bearing and magnetic device orientation (after any deviation corrections) to a range of 0 - 359 degrees, where 0deg is due north
>
>2. Direction of arrow on device screen, in degrees:
>

>ArrowDirection = (GPS Bearing) - (Device Orientation)
>
>IF ArrowDirection < 0
>  ArrowDirection = ArrowDirection + 360
>
>ENDIF
>

I considered that one but thought there would be a way without converting both bearing and heading to 360deg values.

My testing so far seems to show that this simplified versions would work:

ArrowDirection = 360 + (Bearing - Orientation)

In practice it actually makes more sense for me to convert the DeviceOrientation to a 360deg value earlier on so it just becomes:

ArrowDirection = Bearing - Orientation

In either case I don't think ArrowDirection will ever be > 360 so that removes another step.

To be absolutely accurate I need to factor in the declination but that turns out to be simple - there's a built in android function:

GeomagneticField geoField = new GeomagneticField(
                        Double.valueOf(location.getLatitude()).floatValue(),
                        Double.valueOf(location.getLongitude()).floatValue(),
                        Double.valueOf(location.getAltitude()).floatValue(),
                        System.currentTimeMillis()
                );
float declination = geoField.getDeclination;

After that all I have to worry about is the Surface orientation - but I might just lock to the default and not worry about it (at least for now)