Spatial selection function of the thin-disk RC sample. Top: observed (green histogram) and predicted (blue curves) number density laws of a complete sample in R bins. Modelled profiles with the disk warp taken into account are shown in dashed blue. Both observed and modelled density laws are normalised to the midplane densities. Bottom: z weights calculated as a ratio of the observed to modelled profiles. The grey line marks ω_z = 1, the case of a complete sample.