Colour–magnitude diagram K_s vs. J − K_s of common stars between the photometric and the proper motion catalogues. The colour code corresponds to stellar densities using a power stretch scale. The blue dashed boxes indicate the stars belonging to the NSD and the NSC following the different extinction criterion. Right panels show the uncertainties of the proper motions μ_l (Galactic longitude coordinate) and μ_b (Galactic latitude coordinate). The blue dashed lines indicate the 90th percentile of the data (that are below 0.7 mas yr⁻¹ in both cases).