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Our analysis uses diverse data sets, but it hinges on combining those data sets with a modern understanding of the meaning of Galactic coordinates. When, in 1959, the IAU established the current system of Galactic $(l,b)$ coordinates \citep{Blaauw1959}, the positions of the Sun with respect to the ``true" Galactic disk, and of the Galactic Center, were not as well determined as they are now. As a result, the Galactic Plane is typically \textbf{not} at $b=0$, as  projected onto the sky. The exact offset from $b=0$ depends on distance, as we explain in \S \ref{lookingdown}. Taking these offsets into account, one can profitably re-examine data relevant to the Milky Way's 3D structure. The Sun's vantage point slightly ``above" the plane of the Milky Way offers useful perspective.  ``IRDCs" are loosely defined as clouds with column densities high enough to be obvious as patches of significant extinction against the diffuse galactic background mid--infrared wavelengths. \citet{Peretto2009a} set the boundaries of IRDCs at an optical depth of 0.35 at $8~\rm{}\mu{}m$ wavelength, equivalent to an $\rm{}H_2$ column density $\approx{}10^{22}~\rm{}cm^{-2}$. In the \citet{peretto2010:irdcs-mass-density} sample, clouds have average column densities of a few $10^{22}~\rm{}cm^{-2}$. Some IRDCs actively form high--mass stars (e.g., \citealt{pillai2006:g11} and \citealt{rathborne2007:irdc-msf}). \citet{kauffmann2010:irdcs} explain that while some ``starless" IRDCs are potential sites of future high--mass star formation, and the few hundred densest and the most massive, IRDCs may very well contain a large fraction of the star--forming gas in the Milky Way, it is still true that most IRDCs are not massive and dense enough to form high--mass stars. Thus, a small number of very dense and massive IRDCs may be responsible for a large fraction of the galactic star formation rate, and an extragalactic observer of the Milky Way might ``see" IRDCs not unlike Nessie  hosting young massive stars as the predominant mode of star formation here. The traditional ISM-based probes of the Milky Way's structure have been HI and CO. Emission in these tracers gives line intensity as a function of velocity, so the position-position-velocity data resulting from HI and CO observations can give three dimensional views of the Galaxy, if a rotation curve is used to translate line-of-sight velocity into a distance. Unfortunately, though, the Galaxy is filled with HI and CO, so it is very hard to disentangle features when they overlap in velocity along the line of sight. Nonetheless, much of the basic understanding of the Milky Way's spiral structure we have now comes from HI and CO observations of the Galaxy, much of it from the compilation of CO data presented by \citet{Dame2001}.