Systematic velocity trends induced by the simulated uncertainties in our mock catalogue. Upper left panel: map of the observed V_R assuming a relative parallax uncertainty = 0.2. Upper right panel: same as the left panel, but showing the observed ΔV_ϕ. Lower left panel: observed V_R assuming that the true value of R_⊙ is 200 pc higher than the adopted value, together with the distance uncertainty model explained in the text. Lower right panel: observed V_R assuming that the true value of the solar velocity component V_ϕ, ⊙ is 10 km s⁻¹ higher than the adopted value, together with the distance uncertainty model explained in the text.