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Published online by Cambridge University Press: 14 August 2015
Although radio recombination line and continuum observations are very useful for investigating galactic structure, it is well to remember their limitations. First, they only provide measurements of coordinates and velocities; a kinematic model is needed to derive the distance and thus the actual location of every nebula. In some directions, particularly all longitudes from ℓ=0°, kinematic distance estimates are prone to systematic errors arising from velocity crowding or uncertainties in the rotation curve, and are of little use in quantitative studies. I will only discuss the distribution of radio nebulae in the inner ∼100° of the galactic plane, since in this area kinematic analyses can give reasonable results and, in any case, here we must rely on radio observations for most of our information.