Best-fitted models of polynomial-logarithmic (PL) spirals to only the HII regions (left) and all three kinds of spiral tracers (right). The symbols are the same as those in Fig. 2. The dashed-lines indicate the observed tangential directions (see Table 2). In the far end of the GC direction, the spiral tracers generally have large position error bars (see e.g., Fig. 5), resulting in the large uncertainties of the fitted positions of arm segments which are indicated as dashed-lines for Galaxy longitude range of 340° ≤ l ≤ 20°.