Create Square Overlay that covers current region in MKMapView

How can I create a Square Overlay that will cover the current region of my MKMapView. My MKMapView is set with coordinates of the users current location but they are centre coordinates. How do I calculate the square coordinates so I can create a square overlay that fits perfectly in the current view?

Thanks guys!

You can use the centerCoordinate and region properties of MKMapView and then create a MKPolygon overlay by extracting the four corners, as in the code below:

    vertex[0]=CLLocationCoordinate2DMake(map.centerCoordinate.latitude+map.region.span.latitudeDelta/2.,map.centerCoordinate.longitude-map.region.span.longitudeDelta/2.);
    vertex[1]=CLLocationCoordinate2DMake(map.centerCoordinate.latitude+map.region.span.latitudeDelta/2.,map.centerCoordinate.longitude+map.region.span.longitudeDelta/2.);
    vertex[2]=CLLocationCoordinate2DMake(map.centerCoordinate.latitude-map.region.span.latitudeDelta/2.,map.centerCoordinate.longitude+map.region.span.longitudeDelta/2.);
    vertex[3]=CLLocationCoordinate2DMake(map.centerCoordinate.latitude-map.region.span.latitudeDelta/2.,map.centerCoordinate.longitude-map.region.span.longitudeDelta/2.);
    MKPolygon *square = [MKPolygon polygonWithCoordinates:vertex count:4];

Then you add the polygon as an overlay:

[map addOverlay:square]

Finally in your mapView:rendererForOverlay: you define your square rendered based on the polygon overlay:

- (MKOverlayRenderer *)mapView:(MKMapView *)mapView rendererForOverlay:(id)overlay {  
    if([overlay isKindOfClass:[MKPolygon class]]) {
        MKPolygonRenderer *renderer = [[MKPolygonRenderer alloc] initWithPolygon:(MKPolygon *)overlay];
        renderer.fillColor = [[UIColor redColor] colorWithAlphaComponent:0.25];
        return renderer;
    } else {
        return nil;
    }    
}