Velocity maps for three photospheric heights, showing the atmospheric vertical velocity v _z at a given geometrical height and the mean velocity shifts Δ v derived from several photospheric lines. The color scale is clipped at −3 km s ⁻¹ and 3 km s ⁻¹ to maximize contrast. The middle panels shows Δ v when no noise was applied on the spectra and the right panels show the same quantity when Poisson noise was added with a S/N of 10 and a Wiener filter was used on the data (see the text).