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.
