How can I get the magnetic field vector, independent of the device rotation?

Question

What I want to archieve is a sort of "magnetic fingerprint" of a location. I use the MAGNETIC_FIELD sensor and in the event I get the 3 values for the (unfortunately not further explained) X, Y and Z axis.

Problem is, that the values change as I rotate the device, so I guess the 3 axis are relative to the device. What I'd need is to compensate the device rotation so that I get the same 3 values, regardless of how the device is rotated.

I tried to multiply with the rotation matrix (I know how to get that), tried to multiply with the inclination matrix and so on, but nothing works. Regardless of what I try, still the values change when I rotate the device.

So does anyone know how to do it right? Preferrably with code, because I read a lot of stuff like 'well then you'll have to compensate that using rotation matrix' but did not find a single concrete, working example.

Answer 1

Do this

private static final int TEST_GRAV = Sensor.TYPE_ACCELEROMETER;
private static final int TEST_MAG = Sensor.TYPE_MAGNETIC_FIELD;
private final float alpha = (float) 0.8;
private float gravity[] = new float[3];
private float magnetic[] = new float[3];

public void onSensorChanged(SensorEvent event) {
    Sensor sensor = event.sensor;
    if (sensor.getType() == TEST_GRAV) {
            // Isolate the force of gravity with the low-pass filter.
              gravity[0] = alpha * gravity[0] + (1 - alpha) * event.values[0];
              gravity[1] = alpha * gravity[1] + (1 - alpha) * event.values[1];
              gravity[2] = alpha * gravity[2] + (1 - alpha) * event.values[2];
    } else if (sensor.getType() == TEST_MAG) {

            magnetic[0] = event.values[0];
            magnetic[1] = event.values[1];
            magnetic[2] = event.values[2];

            float[] R = new float[9];
            float[] I = new float[9];
            SensorManager.getRotationMatrix(R, I, gravity, magnetic);
            float [] A_D = event.values.clone();
            float [] A_W = new float[3];
            A_W[0] = R[0] * A_D[0] + R[1] * A_D[1] + R[2] * A_D[2];
            A_W[1] = R[3] * A_D[0] + R[4] * A_D[1] + R[5] * A_D[2];
            A_W[2] = R[6] * A_D[0] + R[7] * A_D[1] + R[8] * A_D[2];

            Log.d("Field","\nX :"+A_W[0]+"\nY :"+A_W[1]+"\nZ :"+A_W[2]);

        }
    }