- (void)mapView:(MKMapView *)mapView regionDidChangeAnimated:(BOOL)animated {
for (id <MKAnnotation>annotation in _mapView.annotations) {
    // if it's the user location, just return nil.
    if ([annotation isKindOfClass:[MKUserLocation class]])
        continue;

    // handle our custom annotations
    //
    if ([annotation isKindOfClass:[XPMKAnnotation class]])
    {
        // try to retrieve an existing pin view first
        MKAnnotationView *pinView = [_mapView viewForAnnotation:annotation];
        //resize the pin view
        double zoomLevel = [_mapView getZoomLevel];
        double scale = (1.0 * zoomLevel / 16) + 0.5;
        pinView.transform = CGAffineTransformMakeScale(scale, scale);
    }
    }
}