Systematic uncertainties when using Jeans equations to calculate υ_c. Lines show the difference between different selections or methods and the fiducial BJ model. The light green (dark green) solid line shows the systematic shift when changing the exponential scale length of the number density by −1 kpc (1 kpc). The red (blue) dash-dotted line shows the variation expected considering samples below (above) the disc, selecting ɀ < 0 (ɀ > 0) only. The red dotted line shows the influence of the crossterm in Eq. (6). The dashed purple and orange lines show Δυ_c/υ_c for samples for which −30 < ϕ < 0° and 0 < ϕ < 30° respectively. The dotted grey line shows the influence of trying to fit $\langle v_R^2\rangle$ to its behaviour in the outskirts, resulting in $R_{\exp ,\langle v_{\text{R}}^2\rangle }=43\text{\hspace{0.17em}kpc}$. The black line guides the eye to zero difference with the fiducial model.