Three-dimensional (l, b, D) diagram for the W3–W4 regions. 478 (43%) sources are observed in the range 1.8–2.3 kpc, the range of distances obtained by Zucker et al. (2020) for these two star forming regions. The color coding is the same as in Fig. 13, that is, sources in the range 1.8–2.3 kpc are coded in green, sources outside this range are coded in red, and unfitted sources (with no distance information) are coded in blue; the distance layer for these latter sources is put arbitrarily at 3 kpc for clarity in the plot.