title: How is the altitude of MKMapCamera calculated?
body: <p>I've been logging the altitude of an <code>MKMapView</code>'s camera as the view is scrolled and I've discovered a strange behavior: the altitude of the viewpoint fluctuates as the user scrolls although the apperance of the map's zoom level stays the same.</p>
  
  <p>After closer inspection, it seems that the altitude decreases as the user scrolls South (toward the equator) and the increases again once the user passes the equator. I thought that it might have something to do with curvature of the Earth and possible involve some trigonometry or cartography knowledge that I don't have.</p>
  
  <p>I am trying to emulate this function. I have a <code>CLLocationCoordinate2D</code> and the current altitude and zoom level and I want to calculate the proper altitude for the <code>MKMapCamera</code>.  </p>
  
  <p>Thanks!</p>