Rotation speed derived from Eq. (8) as a function of R for different z, with Δz = 400 pc; correction of systematic errors of proper motions included; adopting a solar azimuthal speed of V_g, ⊙ = 250 km s^-1 and a LSR azimuthal speed of V_c(R_⊙,z = 0) = 238 km s^-1. Error bars include statistical errors, uncertainties in (U_⊙,V_g, ⊙,W_⊙), and errors of the systematic errors of proper motion and contamination of non-RCGs (see Fig. 4 for the distribution of errors for z = 0).