title: Is the iPhone accelerometer calibrated? Gravity measurement changes depending on orientation
body: <p>I'm doing some test with iPhone 4S accelerometer. If I take the raw data in Z-axis (telephone rest over desktop) I get an acceleration 9.65-9.70 m/s2 (after g conversion by 9.8261).
  But if i have the telephone resting over edge, the measurement of the accelerometer value in the X-axis is so different, aprox. 9.80-9.85 m/s2 (after the same g conversion).
  My question is, if the gravity is the same, why this difference? It is not callibrated?</p>
  
  <p>On the other hand, I check the module value at both situations and the difference is the same.
  Thanks.</p>