In geology, what is apparent dip and how is it related to true dip?
From https://en.wikipedia.org/wiki/Strike_and_dip


  Apparent dip is the name of any dip measured in a vertical plane that is not perpendicular to the strike line. True dip can be calculated from apparent dip using trigonometry if you know the strike. Geologic cross sections use apparent dip when they are drawn at some angle not perpendicular to strike.


see http://www.webpages.uidaho.edu/~simkat/geol345_files/2012lab2.html