Nonflat surface topography introduces a numerical problem for migration algorithms that are based on depth extrapolation. Since the numerically efficient migration schemes start at a flat interface, wave-equation datuming is required (Berryhill, 1979) prior to the migration. The computationally expensive datuming procedure is often replaced by a simple time shift for the elevation to datum correction. For nonvertically traveling energy this correction is inaccurate. Subsequent migration wrongly positions the reflectors in depth.