The start of the Sagittarius spiral arm (Sagittarius origin) and the   start of the Norma spiral arm (Norma origin) - model-computed and observed   arm tangents at galactic longitudes -20 degrees < l < +23 degrees
  Here we fitted a 4-arm spiral structure to the more accurate data on global arm pitch angle and arm longitude tangents, to get the start of each spiral arm near the Galactic nucleus. We find that the tangent to the 'start of the Sagittarius' spiral arm (arm middle) is at l= -17 degrees +/- 0.5 degree, while the tangent to the 'start of the Norma' spiral arm (arm middle) is at l= +20 degrees +/- 0.5 degree.   Earlier, we published a compilation of observations and analysis of the tangent to each spiral arm tracer, from longitudes +23 degrees to +340 degrees; here we cover the arm tracers in the remaining longitudes +340 degrees (=- 20 degrees) to +23 degrees. Our model arm tangents are confirmed through the recent observed masers data (at the arm's inner edge). Observed arm tracers in the inner Galaxy show an offset from the mid-arm; this was also found elsewhere in the Milky Way disk (Vallee 2014c).   In addition, we collated the observed tangents to the so-called '3-kpc-arm' features; here they are found statistically to be near l= -18 degrees +/- 2 degrees and near l= +21 degrees +/- 2 degrees, after excluding misidentified spiral arms. We find that the model-computed arm tangents in the inner Galaxcy are spatially coincident with the mean longitude of the observed tangents to the '3-kpc-arm' features (same galactic longitudes, within the errors). These spatial similarities may be suggestive of a contiguous space. 
Jacques P Vallee
astro-ph.GA