Radial velocity and metallicity maps in R_g − z (left) and R_g − v_z (right) coordinates taken in a narrow ±5° azimuthal selection centred on the Sun-Galactic centre line. The magenta contours in the right panels show 1σ levels of the vertical velocity distribution. Top panels: mean radial velocity maps for the G3RV2. The following rows represent the mean radial velocity, the mean [Fe/H], the mean [Fe/H] for [α/Fe] < 0.1, and the residual (relative to the extracted 2D gradient) [Fe/H] maps for the G3RV2 stars in common with Galah (left), AstoNN (centre), and LAMOST (right). The radial velocity wave-like pattern, correlating with the stellar overdensities in (X_g,Y_g) and (R, v_ϕ) coordinates (see Figs. 5 and 8), shows a large vertical extension. This suggests that the spiral arms and bar resonances can be traced far from the Galactic plane by stars with high vertical velocities. Similarly, the [Fe/H] pattern is detected significantly away from the Galactic plane, consistent with the radial velocity variations.