Island height can be measured with diffraction similar to the way light produces Newton's fringes in an air wedge. By varying the electron energy between constructive/destructive interference from the island top and the area between fringes are produced.
M. Yakes, M. Hupalo, M. A. Zaluska-Kotur, Z. W. Gortel, and M. C. Tringides, Phys. Rev. Lett. 98, 135504 (2007). "Low-Temperature Ultrafast Mobility in Systems with Long-Range Repulsive Interactions: Pb/Si(111)"