A method of measurement of topographic features on a surface of a substrate is presented, wherein a focused beam of particles falls onto the surface of the substrate, and backscattered particles are detected with a particle detector. An opaque material is interposed between the surface and the detector, and the position of the shadow of an edge of the opaque material on the detector is recorded. The relative position of the edge and the surface of the substrate is then determined, and the topography of the surface determined as the particle beam and the substrate are moved with respect to one another.