title: How does my iPhone get its altitude?
body: <p>I made a simple app which displays the altitude according to the GPS position on iPhone. </p>
  
  <ul>
  <li><p>How is the altitude obtained?</p></li>
  <li><p>Does it get the altitude of the land at the coordinates I am or does it get the actual altitude? (I tried the same application in the same building; once at ground floor and once on the third floor but the same altitude is displayed)</p></li>
  <li><p>Is it possible to obtain a different altitude on different floors? (Like my android phone has a barometer sensor and I calculate the elevation by the atmospheric pressure; but none of the iPhones has a barometer sensor)</p></li>
  </ul>