This paper discusses various techniques to implement the physical-optics approximation for topography simulation. The accuracies of the various techniques are examined by comparison with the results of a rigorous, time- domain method. It is shown that inter-surface multiple scattering effects and near-field diffraction effects in intra-surface multiple scattering must both be taken into account to obtain satisfactory agreement with the time-domain method. A technique to correct for the effects of non-physical-optics edge currents is also described.