This video explains the trigonometric proof for how we can calculate the true dip of a plane if we have information about the apparent dip.
Bibliography for picture and proof:
Rowland, Stephen Mark, Ernest M. Duebendorfer, and Ilsa M. Schiefelbein. "Attitudes of Lines and Planes." Structural Analysis and Synthesis. Malden (MA): Blackwell, 2007. 6-8. Print.