Panel A: face-on view of the UMS P18 data set in the Galactic disc. The position of the Sun is shown by the white cross in (X, Y) = (0, 0). The Galactic centre is to the right, at (X, Y) = (R_⊙, 0), and the Galaxy is rotating clockwise. Panel B: same as panel A, but showing the measured overdensity using the UMS P18 data set, based on a local density scale length 0.3 kpc. Only points with Σ(x, y) > 0.003 are plotted in order to remove regions where the statistics is too low. The corresponding plot for the UMS Z21 sample can be found in Fig. B.1. Panel C: same as panel A, but showing the wavelet transformation at the scale 3 (size∼0.4 kpc). A different version of panels B and C using a larger scale length can be found in Fig. B.2. The maps shown in panels B and C are publicly available at https://github.com/epoggio/Spiral_arms_EDR3.git.