  Over 100 trigonometric parallaxes and proper motions for masers associated
with young, high-mass stars have been measured with the BeSSeL Survey, a VLBA
key science project, the EVN, and the Japanese VERA project. These measurements
provide strong evidence for the existence of spiral arms in the Milky Way,
accurately locating many arm segments and yielding spiral pitch angles ranging
from 7 to 20 degrees. The widths of spiral arms increase with distance from the
Galactic center. Fitting axially symmetric models of the Milky Way with the 3-D
position and velocity information and conservative priors for the solar and
average source peculiar motions, we estimate the distance to the Galactic
center, Ro, to be 8.34 +/- 0.16 kpc, a circular rotation speed at the Sun, To,
to be 240 +/- 8 km/s, and a rotation curve that is nearly flat (a slope of -0.2
+/- 0.4 km/s/kpc) between Galactocentric radii of 5 and 16 kpc. Assuming a
"universal" spiral galaxy form for the rotation curve, we estimate the thin
disk scale length to be 2.44 +/- 0.16 kpc. The parameters Ro and To are not
highly correlated and are relatively insensitive to different forms of the
rotation curve. Adopting a theoretically motivated prior that high-mass star
forming regions are in nearly circular Galactic orbits, we estimate a global
solar motion component in the direction of Galactic rotation, Vsun = 14.6 +/-
5.0 km/s. While To and Vsun are significantly correlated, the sum of these
parameters is well constrained, To + Vsun = 255.2 +/- 5.1 km/s, as is the
angular speed of the Sun in its orbit about the Galactic center, (To + Vsun)/Ro
= 30.57 +/- 0.43 km/s/kpc. These parameters improve the accuracy of estimates
of the accelerations of the Sun and the Hulse-Taylor binary pulsar in their
Galactic orbits, significantly reducing the uncertainty in tests of
gravitational radiation predicted by general relativity.
