calculate acceleration in reference to true north

Question

For my App I need to calculate the acceleration of my device in reference to true north. My idea was to calculate the device orientation to magnetic north and apply the decl

Answer 1

The accelerometer sensor returns the acceleration of the device. This is a vector in 3 dimentional space. This vector is returned in the device coordinate system. What you want is the coordinate of this vector in the world coordinate, which is simply

R = rotation matrix obtained by calling getRotationMatrix
A_D = accelerator vector return by sensor ( A_D = event.values.clone )
A_W = R * A_D is the same acceleration vector in the world coordinate system.

A_W is an array of dimention 3 
A_W[0] is acceleration due east.
A_W[1] is acceleration due north.

Here is some code to compute it (assumes gravity and magnetic contain output from their respective sensors):

            float[] R = new float[9];
            float[] I = new float[9];
            SensorManager.getRotationMatrix(R, I, gravity, magnetic);
            float [] A_D = 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];