java - How to know if Android device is Flat on table -

i'm using accelometer sensor detect whether device flat on table or not. weird thing when make phone flat or rotate on it's side value between 90 , 100! shouldn't correct! missing something? here code:

   float[] values = event.values;     // movement     float x = values[0];     float y = values[1];     float z = values[2];     float norm_of_g =(float) math.sqrt(x * x + y * y + z * z);      // normalize accelerometer vector     x = (x / norm_of_g);     y = (y / norm_of_g);     z = (z / norm_of_g);     int inclination = (int) math.round(math.todegrees(math.acos(y)));     log.i("tag","incline is:"+inclination);      if (inclination < 25 || inclination > 155)     {         // device flat         toast.maketext(this,"device flat - beep!",toast.length_short);     } 

edit: i'm using code : how measure tilt of phone in xy plane using accelerometer in android

you're using y-axis instead of z-axis used in the answer linked.

the value of acos near-zero when argument near 1 (or near 180 degrees when near negative one), seen in picture:

as such, inclination near 0 (or 180) degrees when y axis normalized 1 or negative one, eg when parallel gravity, (thus, device "standing up").

if there's no other error, switching from:

int inclination = (int) math.round(math.todegrees(math.acos(y))); 

to

int inclination = (int) math.round(math.todegrees(math.acos(z))); 

should it.