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MKMapRect zoomRect = MKMapRectNull;
for (id <MKAnnotation> annotation in mapView.annotations) {
    MKMapPoint annotationPoint = MKMapPointForCoordinate(annotation.coordinate);
    MKMapRect pointRect = MKMapRectMake(annotationPoint.x, annotationPoint.y, 0, 0);
    if (MKMapRectIsNull(zoomRect)) {
        zoomRect = pointRect;
    } else {
        zoomRect = MKMapRectUnion(zoomRect, pointRect);
    }
}
[mapView setVisibleMapRect:zoomRect animated:YES];