Two dimensional projections of the median Z for the two working samples, namely the OB sample (left column) and the RGB sample (right column). The sizes of the bins are 0.32 kpc × 0.64 kpc for the OB sample and 0.16 kpc × 0.32 kpc for the RGB sample. We only plot the bins with at least 15 sources. We mark the position of the Sun with a star. The grey solid lines show the Galactocentric azimuths: 135°, 180°, and 225°, and the black dotted lines show circles at different Galactocentric radii: R = 8, 10, 12, 14 and 16 kpc. The colour scale is different in each sample.