getting midline of surface

forgive me if this has been answered before, im having a brain fart.  basically i have two sets of points which i drew curves through then lofted.  from that surface i want to get the exact midline of the surface.  i attached my definition and an image.  in the actual definition i used sin graphs for the points before i lofted so the midline would need to be an average of those curves.  any help is much appreciated, thanks guys and gals.