地图位置点的圆圈放大效果是怎样弄上去的？该怎么处理

- (MKAnnotationView *)mapView:(MKMapView *)aMapView viewForAnnotation:(id <MKAnnotation>)annotation{    if(![annotation isKindOfClass:[MyAnnotation class]]) // Don't mess user location        return nil;    MKAnnotationView *annotationView = [aMapView dequeueReusableAnnotationViewWithIdentifier:@"spot"];    if(!annotationView)    {        annotationView = [[MKAnnotationView alloc] initWithAnnotation:annotation reuseIdentifier:@"spot"];        annotationView.rightCalloutAccessoryView = [UIButton buttonWithType:UIButtonTypeDetailDisclosure];        [(UIButton *)annotationView.rightCalloutAccessoryView addTarget:self action:@selector(openSpot:) forControlEvents:UIControlEventTouchUpInside];        annotationView.enabled = YES;        annotationView.canShowCallout = YES;        annotationView.centerOffset = CGPointMake(7,-15);        annotationView.calloutOffset = CGPointMake(-8,0);    }    // Setup annotation view    annotationView.image = [UIImage imageNamed:@"pinYellow.png"]; // Or whatever    return annotationView;}