Hello,
If you are using a multiphase model, please create a plane such that it crosses the interface. You can then create Iso-Surfaces of Volume Fraction = 0.5 from these planes. Once you do this, you should be able to plot the y-coordinate (if this represented the elevation) as a function of distance (perhaps, the x-coordinate).
If this does not answer your question, please share a screenshot of what you are modeling and elaborate your question further so we can help.
Thanks.
Best,
Karthik