Examples of bias when using the rotation curve method for a representative model at i = 40° (in red in Fig. 8): “Observed” launch radii r_obs(r) (a) and magnetic lever arms λ_obs(r) (b) inferred at each r by application of Anderson’s formula to j_obs(r) and V_p,obs(r) (Eqs. (25) and (26)). Green curves illustrate the spatially resolved regime (θ_b = 45 au) and red curves the unresolved regime (θ_b = 225 au). These curves stop at 1% of the PV peak intensity, with a plus symbol marking 10%. The black vertical line shows the flow radius r_j. For comparison, dotted blue curves plot the true launch radii r₀(r) (a) and λ_ϕ(r) (b) of each DW streamline tangent to r; horizontal dashed blue lines mark the true outermost launch radius r_out = 8 au (a) and true λ_BP = 5.5 (b) in the model. Other model parameters are: MHD solution L5W17, α = −2, M_⋆ = 0.1 M_⊙, and r_in = 0.25 au.