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\section{Introduction}  Many surprisingly fundamental questions remain about the structure of the Milky Way. For instance, does the Milky Way have two \cite{Jackson_2008,Francis_2009, Dobbs_2012} or four \cite{Reid_2009, Bobylev_2013,Urquhart_2013} major spiral arms? What is the precise location of these arms in position-position-velocity (\textit{p-p-v})  space? And what is the nature of inter-arm structure---is it made of well-defined spurs or more web-like structures? An understanding of the Milky Way's true structure has eluded us, largely due to the fact that we are embedded in the galaxy we are attempting to delineate. Much of our current understanding of the Milky Way's three-dimensional structure stems from radial velocity measurements of gas. Making use of the Milky Way's rotation curve \cite{McClure_Griffiths_2007, Reid_2014}, we can translate line-of-sight velocities into distances, thereby constructing a gross three-dimensional model of our Galaxy. Thanks to a wealth of spectroscopic surveys, velocity-resolved observations are readily available for much of the Galaxy's molecular, atomic, and ionized gas. Extended tracers, like CO \citep{Dame_2001} or HI \citep{Shane_1972} provide the best constraints on the Galaxy's overall anatomy. To probe finer structure, observations of high-mass star forming regions can also provide kinematic and distance information for high-density gas. For instance, measurements of trigonometric parallaxes and proper motions of masers from the BeSSeL survey produced accurate locations for several spiral arm segments, along with their associated pitch angles \citep{Reid_2014}. Likewise, the Bolocam Galactic Plane Survey \citep[BGPS,][]{Schlingman_2011, Shirley_2013, Ellsworth_Bowers_2013, Ellsworth_Bowers_2015}, the Millimetre Astronomy Legacy Team 90 GHz Survey \citep[MALT90,][]{Foster_2011,Jackson_2013}, and the $\textrm{H}_2\textrm{O}$ Southern Galactic Plane Survey \citep[HOPS,][]{Purcell_2012,Walsh_2011} have produced hundreds of high-spectral resolution velocity measurements of the dense gas in molecular clouds. Analyses of extinction data from surveys like Pan-STARRS1 complement this emission line data and can also be used to create three-dimensional models of the Galaxy's structure \citep{Green_2014,Schlafly_2014}.