Vertical residuals, Δ_Z, of the m=1 straight line-of-nodes model using young giants (left panel) and Cepheids (right panel), compared to spiral arm geometries taken from the literature: Reid et al. (2019) (black solid lines, both panels), Poggio et al. (2021) (grey shaded contours, left panel), and Drimmel et al. (2024) (coloured lines, right panel). For the Drimmel et al. (2024) model, different colours indicate the Perseus arm (grey), Local/Orion arm (orange), Sagittarius-Carina arm (blue), and Scutum arm (red).