gistfile1.m

#define kApproxRadiusOfEarthInMiles                     3963.1676
#define kApproxSizeOfOneDegreeLatitudeInMiles           68.71 
#define kApproxSizeOfOneDegreeLongitudeAtLatitude(lat)  ((M_PI/180.0)* kApproxRadiusOfEarthInMiles *cos(lat))

static function MKCoordinateSpan MKCoordinateSpanMakeWithDistanceInMiles(float miles, CLLocationDegrees latitude) {
  MKCoordinateSpan viewSpan;
  viewSpan.latitudeDelta  = miles / kApproxSizeOfOneDegreeLatitudeInMiles; 
  viewSpan.longitudeDelta = miles / kApproxSizeOfOneDegreeLongitudeAtLatitude(latitude);
  return viewSpan;
}

- (void)locationManager:(CLLocationManager *)manager 
    didUpdateToLocation:(CLLocation *)newLocation 
           fromLocation:(CLLocation *)oldLocation {

    MKCoordinateSpan viewSpan = MKCoordinateSpanMakeWithDistanceInMiles(0.5, newLocation.coordinate.latitude);
    MKCoordinateRegion viewRegion = MKCoordinateRegionMake(newLocation.coordinate, viewSpan);
    
    MKCoordinateRegion adjustedRegion = [mapView regionThatFits:viewRegion];
    [mapView setRegion:adjustedRegion animated:YES];
}