iphone: Drawing over MKMapView?

I'm attempting to draw polygons on a mapView. What I did was to add a transparent view on the main view, that matches the bounds of the mapView, and I draw over it. When I move the table it's ok, but when I zoom in and out the polygones 'bounce' on the Y axis (latitude) but the x axis is ok. What do you think?

BTW开发者_开发技巧 I tried to add the transparent view as a subview to the mapView but then it screwed up the user interactions.

How can this be done? Help would be appreciated.

Check out this post. It's a little old but still works. He shows how to draw a route over a MKMapView, which is similar.

The post is: http://spitzkoff.com/craig/?p=65