Using the H I Emission/Absorption method , we resolve the kinematic distance ambiguity and derive distances for 149 of 182 ( 82 % ) H II regions discovered by the Green Bank Telescope H II Region Discovery Survey ( GBT HRDS ) .
The HRDS is an X-band ( 9 GHz , 3 cm ) GBT survey of 448 previously unknown H II regions in radio recombination line and radio continuum emission .
Here we focus on HRDS sources from 67 \arcdeg \geq \ell \geq 18 \arcdeg , 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 20 kpc distant .
They are the most distant H II 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 ” H II regions are not preferentially at the near distance ( as was assumed previously ) but average 10 kpc from the Sun .
The HRDS nebulae , when combined with a large sample of H II regions with previously known distances , show evidence of spiral structure in two circular arc segments of mean Galactocentric radii of 4.25 and 6.0 kpc .
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 H II regions is only 0.5 kpc , or 5 % .
This is significantly less than the median difference between the near and far kinematic distances , 6 kpc .
The basic Galactic structure results are unchanged after considering these sources of uncertainty .