The Green Bank Telescope HII Region Discovery Survey: III. Kinematic
  Distances
  Using the HI Emission/Absorption method, we resolve the kinematic distance
ambiguity and derive distances for 149 of 182 (82%) HII regions discovered by
the Green Bank Telescope HII Region Discovery Survey (GBT HRDS). The HRDS is an
X-band (9GHz, 3cm) GBT survey of 448 previously unknown HII regions in radio
recombination line and radio continuum emission. Here we focus on HRDS sources
from 67deg. > l > 18deg., where kinematic distances are more reliable. The 25
HRDS sources in this zone that have negative recombination line velocities are
unambiguously beyond the orbit of the Sun, up to 20kpc distant. They are the
most distant HII regions yet discovered. We find that 61% of HRDS sources are
located at the far distance, 31% at the tangent point distance, and only 7% at
the near distance. "Bubble" HII regions are not preferentially at the near
distance (as was assumed previously) but average 10kpc from the Sun. The HRDS
nebulae, when combined with a large sample of HII regions with previously known
distances, show evidence of spiral structure in two circular arc segments of
mean Galactocentric radii of 4.25 and 6.0kpc. We perform a thorough uncertainty
analysis to analyze the effect of using different rotation curves, streaming
motions, and a change to the Solar circular rotation speed. The median distance
uncertainty for our sample of HII regions is only 0.5kpc, or 5%. This is
significantly less than the median difference between the near and far
kinematic distances, 6kpc. The basic Galactic structure results are unchanged
after considering these sources of uncertainty.
