Frequency map in cylindrical coordinates for z-tubes alone (orbits for which a circulation around the z-axis has been detected), i.e. orbits from the resonance (1:−1:0) in Cartesian coordinates (see Fig. 8). The horizontal and vertical lines correspond to resonances between Ω_z and Ω_R and between Ω_ϕ and Ω_R respectively. The typical error on both frequency ratios arising from the propagation of the observational uncertainties and the distance uncertainty is indicated in the lower right corner.