I am trying to find the angle between two surfaces. Please see screenshot attached. I have created construction line so that i can find an angle between the two vectors but it is not giving me the right answer, so therefore i wondered if there is a way of finding angle between surfaces directly.
Can someone please assist.

The angle between two planar surfaces is identical to the angle between their normal vectors. There’s a component for vector|vector angle, you just have to make sure that they both point in the right direction.

David’s answer is far simpler than anything I can offer.

Wow, that’s a lot of code! Took awhile to locate what might be the flaw? The blue lines connected to group “Theta Angle Between Two Planes” are not the angle between them, eh? Where the points are coming from that define those lines is even harder to trace. So I won’t.

I tried to measure the angle between 2 surfaces. And I noticed there is 180 difference between the grasshopper and the Rhino DimCreaseAngle, sometimes, not always .
I tried your script and i have the same problem.
Maybe you know why that may happened?