[Ca/Fe] chemical inhomogeneities in the Galactic disc for samples A and C. Left panel: map of [Ca/Fe] abundances for sample A. The black contours indicate the position of the spiral arms obtained using upper main sequence stars from Poggio et al. (2022) that trace from left to right the Perseus arm, the Local arm, and the Sagittarius-Carina arm. The position of the Sun is shown by the black cross at the centre. The Galactic centre is to the right and the Galactic rotation is clockwise. Rings of a constant Galactocentric radius are overplotted as solid grey lines. Right panel: same as the left panel, but for sample C and using the position of the spiral arms found in the subsample of giant stars in Palicio et al. (2023) as black contours. A weak signature of the Gaia scanning law mentioned in the selection function (cf. Appendix A) is still visible, particularly around X ~ 0 kpc and Y = (−4.5, −1) kpc or Y = (2, 4.5) kpc.