Heliocentric velocity residual maps for Δ(−V_ϕ) (left) and ΔV_R (right) for the stars (top) and gas (bottom) in the mid-slice of our model (|z| < 0.25 kpc). The ridges of the spiral arm segments extracted from the sharpened images of Fig. C.1, are superimposed for comparison – grey for gaseous and black for stellar ridges. The Galactic centre is at (8.275,0), and the Sun’s position is marked with a black dot. The different cross-sections studied in Section 5.1 are represented with white arrows.