Oh goodie! responses and hints - great! Unfortunately I have a meeting with some alpacas in the Rockies today:)

My suggestion isnt a true intersection, it's based on a iso-height line. Not too difficult.
If I had access to vue source it would barely be a page of programing, without it there is some overhead.
Oh dear - gotta run, see ya later

"I paint that which comes from the imagination or from dreams, or from an unconscious drive. I photograph the things that I do not wish to paint, the things which already have an existence."
Man Ray, modernist painter
http://artpearl.redbubble.com/