title: Calculate area covered on map
body: <p>i'm working on an app that contains a map. Over the map there is a circle that should resemble the covered area in miles. The circle doesn't move around, however the map does move behind it as the user drags it. I want to be able to calculate the area covered by the circle on the map even if the user zoomed in or out. I know it's all mathematical. As an example to what i was talking about, check the app RadPad. Anyway, here is what i have done so far:</p>
  
  <ol>
  <li><p>Calculate the zoom level (as it is needed when user zooms in/out):</p>
  
  <pre><code>- (NSUInteger) zoomLevel:(MKMapView*)map 
  {
      return (21 + round(log2(map.region.span.longitudeDelta * MERCATOR_RADIUS * M_PI / (180.0 * map.bounds.size.width))));
  }
  </code></pre></li>
  <li><p>In mapView:regionDidChangeAnimated i entered the following:</p>
  
  <pre><code>// Place a single pin
  
  MKPointAnnotation *annotation2 = [[MKPointAnnotation alloc] init];
  
  
  [annotation2 setCoordinate:CLLocationCoordinate2DMake(mapView.region.center.latitude, mapView.region.center.longitude-(0.0255*([self zoomLevel:mapView]/12)))];
  
  [_map addAnnotation:annotation2];
  </code></pre></li>
  </ol>
  
  <p>The annotation is used in order to see if i can locate a point on the circumference of the circle.</p>
  
  <p>running this code works fine on default, however when i zoom in / out the annotation is placed inside the circle or somewhere it shouldn't be.</p>
  
  <p>failing to locate a point on the circle will be an obstacle for me to even calculate the area covered. Any help? hint?</p>