Вы можете сделать так ( См. Этот вопрос ):
MKMapRect mapRect = self.mapView.visibleMapRect;
MKMapPoint cornerPointNE = MKMapPointMake(mapRect.origin.x + mapRect.size.width, mapRect.origin.y);
CLLocationCoordinate2D cornerCoordinateNE = MKCoordinateForMapPoint(cornerPointNE);
MKMapPoint cornerPointSW = MKMapPointMake(mapRect.origin.x, mapRect.origin.y + mapRect.size.height);
CLLocationCoordinate2D cornerCoordinateSW = MKCoordinateForMapPoint(cornerPointSW);