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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.  [xxthis paragraph needs editingxx]  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. This implies that IRDCs are relatively starless: if they were to contain bright IR sources and nebulosity, they would not stand out as dark clouds. To give examples, \citet{Peretto2009a} draw 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 their sample, clouds have average column densities of a few $10^{22}~\rm{}cm^{-2}$ (e.g., Figure~2 of \citealt{peretto2010:irdcs-mass-density}). Some IRDCs do actively form high--mass stars (e.g., \citealt{pillai2006:g11} and \citealt{rathborne2007:irdc-msf}). It is therefore thought that some IRDCs are future sites of high--mass star formation. \citet{kauffmann2010:irdcs} demonstrate that most IRDCs are not massive and dense enough to form high--mass stars \citep{kauffmann2010:irdcs}. Still, they also argue that the few $10^2$ most massive and dense clouds may contain a large fraction of the star--forming gas in the Milky Way. In that case, a small number of very dense and massive IRDCs may be responsible for a large fraction of the galactic star formation rate. The massive stars forming in these dense IRDCs are so bright, that extragalactic observers of the Milky Way might see IRDCs hosting young massive stars as the predominant mode of star formation here. Thus, if one can deduce the pattern of IRDCs that an observer outside the Milky Way would see, one can determine the Milky Way's (non-dark-matter) structure, from inside. (CB Note: these last 2 sentences are interesting -- does it contradict Jens' claims made about star forming efficiencies in IRDCs? Jens: changed text to highlight that high--mass SF is limited to the most massive and dense IRDCs. Then the argument still holds that bright stars may trace IRDCs. If this is to hold for Nessie, this cloud should exceed the mass--size limit from \citep{kauffmann2010:irdcs}.) 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}.