If you use pcolor() instead of surf() then it will give you the overhead view like that, and it will make all of the z values 0. That makes it easy to just plot() on top of the pcolor()
To plot a line on top of a surf() plot, you need to use plot3() specifying a z coordinate that is greater than any z coordinate of the data. If you accidentally use 0 as the z coordinate or use plot() instead of plot3(), then the line would be hidden behind any part of the surface that had a positive z value.
