  From the sun's location in the Galactic disk, one can use different arm tracers (CO, HII, thermal or ionized or relativistic electrons, masers, cold or hot dust, etc) to locate a tangent to each spiral arm in the disk of the Milky Way galaxy. We present a Master catalog of the astronomically observed tangents to the Galaxy's spiral arms, using different arm tracers from the literature. Some arm tracers can have slightly divergent results from several papers, so a mean is taken - see Appendix for CO, HII, and masers. The Master catalog of means currently consists of 63 mean tracer entries, spread over many arms (Carina, Crux-Centaurus, Norma, Perseus origin, near 3-kpc, Scutum, Sagittarius), stemming from 107 original arm tracer entries.   Additionally, we updated and revised a previous a previous statistical analysis of the angular offset and linear separation from the mid-arm, for each different mean arm tracer. Given enough arm tracers, and summing and averaging over all spiral arms, one could determine if arm tracers have separate and parallel lanes in the Milky Way. This statistical analysis allows a cross-cut of a galactic spiral arm to be made, confirming a recent discovery of a linear separation between arm tracers. Here, from the mid-arm's CO to the inner edge's hot dust, the arm half-width is about 340 parsecs; doubling would yield a full arm width of 680 parsecs. We briefly compare these observations with the predictions of many spiral arm theories, notably the density wave theory. 
Catalog of observed tangents to the spiral arms in the Milky Way galaxy