are given by $33^\circ$, $43^\circ$, and $12^\circ$ in @Grosbol+2004, @Seigar+2006, and @Davis+2012, respectively. Secondly, there is a lack of statistical studies on the relation between the pitch angle and the amplitude of the stellar spiral arms. Although some statistical studies have used 2D FFT to analyze the pitch angle of the spiral arms, they do not include examination of the amplitude or contrast of the spiral arms [e.g., @Seigar+2005; @Seigar+2006; @Davis+2012]. Thus, further progress in understanding the spiral dynamics requires additional statistical and robust observation data of the morphological properties (both the pitch angle and amplitude) of the stellar spiral arms. This relation between the pitch angle and amplitude will form one of the tests for spiral genesis theories along with the pitch angle-shear rate (or Hubble-type) relation [@Roberts+1975; @SeigarJames1998; @Hozumi2003; @Grand+2012c], existence of systematic angular offset between the young stellar component and the stellar spiral arm [@Fujimoto1968; @Roberts1969; @Egusa+2009; @Foyle+2011; @Ferreras+2012], and the radial dependence of the pattern speed [@Meidt+2008; @Meidt+2009].

Finally, we comment on grand-design spirals. In a series of studies [@Fujii+2011; @Wada+2011] including this one, we have examined the stellar dynamics of non-steady stellar spirals, as well as the interactions between them and the ISM. However, these 