Accuracy of the reconstruction of the bar potential. Left panel (top): estimate of the bar potential from Sect. 2.4 at the Sun’s position in the plane with 41 complex Fourier coefficients in Eq. (10) and the reconstruction using cubic splines. The vertical line denotes the true value. The value in the top right inset denotes the true value in physical units. The potential is always estimated at the same configuration space location (within the plane, at the Sun’s position) but for different velocities. Left panel (bottom): relative accuracy compared to the maximum value of the bar potential at the Sun’s radius denoted in the top right inset. Right panel: Same, but at z = 0.3 kpc with 231 complex Fourier coefficients. The typical accuracy is well below the per cent level, although with a slight bias towards lower amplitudes (i.e. |Φ_{b estimated}|< |Φ_b input|) above the plane.