We applied the currently most comprehensive version of the statistical-parallax technique to derive kinematical parameters of the maser sample with 136 sources .
Our kinematic model comprises the overall rotation of the Galactic disk and the spiral density-wave effects .
We take into account the variation of radial velocity dispersion with Galactocentric distance .
The best description of the velocity field is provided by the model with constant radial and vertical velocity dispersions , ( \sigma U 0 , \sigma W 0 ) \approx ( 9.4 \pm 0.9 ~ { } , 5.9 \pm 0.8 ) ~ { } km / s .
We compute flat Galactic rotation curve over the Galactocentric distance interval from 3 to 15 kpc and find the local circular rotation velocity to be V _ { 0 } \approx ( 235 - 238 ) Â km/s \pm 7 Â km/s .
We also determine the parameters of the four-armed spiral pattern ( pitch angle i \approx ( -10.4 \pm 0.3 ) ^ { \circ } and the phase of the Sun \chi _ { 0 } \approx ( 125 \pm 10 ) ^ { \circ } ) .
The radial and tangential spiral perturbations are about f _ { R } \approx ( -6.9 \pm 1.4 ) Â km/s , f _ { \Theta } \approx ( +2.8 \pm 1.0 ) Â km/s .
The kinematic data yield a solar Galactocentric distance of R _ { 0 } \approx ( 8.24 \pm 0.12 ) ~ { } kpc .
Based on rotation curve parameters and the asymmetric drift we Infer the exponential disk scale H _ { D } \approx ( 2.7 \pm 0.2 ) Â kpc under assumption of marginal stability of the intermediate-age disk , and finally we estimate the minimum local surface disk density , \Sigma ( R _ { 0 } ) > ( 26 \pm 3 ) ~ { } M _ { \odot } pc ^ { -2 } .