Intersection angle between two planes?

All-

A little help?

I have a freeform surface which has been divided by a number of U,V points. Each of these points naturally has a plane associated with it, which is 'in plane' with the freeform surface where each U,V, point is created.

I am trying to determine the angle of each of these planes off of a world (flat) X,Y plane. For instance, in the attached image, all of the points labeled '1' are nearly 90° off of the base X,Y. But exactly how much?

I'm looking for some way to intersect the two planes and dimension the resulting angle.

Thanks a ton.