  To estimate the parameters of the Galactic spiral structure - namely, the
pitch angle i and the number of spiral arms,- data on Galactic masers with
known trigonometric parallaxes were used. We applied the well-known method
based on analysis of the "position angle - distance logarithm" diagram.
Estimates of the pitch angle i obtained from four segments of different arms
belonging to the global Galactic structure are self-consistent and close to
i=-13+/-1 degrees. The segment which is most interesting is that of the Outer
arm. It contains only three masers. Hence, in order to obtain correct
estimates, we also used the data on 12 very young star clusters with distances
determined by Camargo et al. from infrared photometry. The estimates obtained
allow us to conclude in favor of the four-armed model of the Galactic spiral
structure.
