see-thru tile problems

I have this terrain editor that I made and it’s based off triangle tiles right, so when you look at it from one direction and when u look at it from the back all the tiles are see thru, sort of, i took a picture… here is from the back, which is f’d up:
and here is from the front which is perfectly fine:
if anyone knows why it’s doing this I would greatly appreciate it, thanks.

here are some more pics for you to ponder over, first set looking from back to front, second set looking from front to back with the same map…

Anyone have a clue?

Did you enable Z testing and update? Did you request a depth buffer when creating your context/pixel format?

This is a cross post.

See the other incarnation , loads of stuff over there