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Based on kinematic data on masers with known trigonometric parallaxes andmeasurements of the velocities of HI clouds at tangential points in the innerGalaxy, we have refined the parameters of the Allen-Santillan model Galacticpotential and constructed the Galactic rotation curve in a wide range ofGalactocentric distances, from 0 to 20 kpc. The circular rotation velocity ofthe Sun for the adopted Galactocentric distance $R_0=8$ kpc is $V_0=239\pm16$km s$^{-1}$. We have obtained the series of residual tangential, $\DeltaV_{\theta}$, and radial, $\Delta V_R,$ velocities for 73 masers. Based on theseseries, we have determined the parameters of the Galactic spiral density wavesatisfying the linear Lin.Shu model using the method of periodogram analysisthat we proposed previously. The tangential and radial perturbation amplitudesare $f_\theta=7.0\pm1.2$ km s$^{-1}$ and $f_R=7.8\pm0.7$ km s$^{-1}$,respectively, the perturbation wavelength is $\lambda=2.3\pm0.4$ kpc, and thepitch angle of the spiral pattern in a two-armed model is$i=-5.2^\circ\pm0.7^\circ$. The phase of the Sun $\chi_\odot$ in the spiraldensity wave is $-50^\circ\pm15^\circ$ and $-160^\circ\pm15^\circ$ from theresidual tangential and radial velocities, respectively.

Galactic Rotation Curve and Spiral Density Wave Parameters from 73 Masers