Trying to extract upper and lower surface Cp values from a slice


I am working on creating a plot of Cp against x, I have extracted the surface, put in the equation of Cp and sliced it, but I am not really sure how to differentiate between the upper and lower surface values.

This my Cp contour and I am trying to recreate something like this…

Screenshot 2021-04-27 at 1.56.40 pm

Any help would be much appreciated!