How to get Android phone orientation matching human orientation?

Question

I\'m making a map app, including the location arrow that shows you which way you\'re facing, like so:

Answer 1

Did you call remapCoordinateSystem? Otherwise, you only get the right facing value when the phone is hold vertically. For the case When the phone is held so the screen's facing is level with the horizon, there is no way you can get the user's facing. Because to get the facing you have to project the z value of the of the sensor reading into the xy plane in the world coordinate and it is zero when the device is held horizontally.

To be more precise, if you want to get the phone facing then the phone has to be inclined at least about 25 degrees from horizontal and you have to call remapCoordinateSystem. The following code will give you what you want for the last 2 pictures above.
Code

float[] rotationMatrix = new float[9];  

if(SensorManager.getRotationMatrix(rotationMatrix, null, lastAcceleration, lastMagneticField)){
    float[] orientMatrix = new float[3];
    float remapMatrix = new float[9];
    SensorManager.remapCoordinateSystem(rotationMatrix, SensorManager.AXIS_X, SensorManager.AXIS_Z, remapMatrix);
    SensorManager.getOrientation(remapMatrix, orientMatrix);

    orientation = orientMat[0]*180/(float)Math.PI;
}  

The getOrientation gives you the correct values assuming the phone is laying flat. Thus, if the phone is held vertically, then you have to remap coordinate to get the flat position. Geometrically, you project the the phone -z axis down to the world xy plane and then calculate the angle between this projection vector and the world y-axis.