The morphology and kinematics of the spiral structure of the Milky Way are long-standing problems in astrophysics .
In this review we firstly summarize various methods with different tracers used to solve this puzzle .
The astrometry of Galactic sources is gradually alleviating this difficult situation caused mainly by large distance uncertainties , as we can currently obtain accurate parallaxes ( a few \upmu as ) and proper motions ( \approx 1 km s ^ { -1 } ) by using Very Long Baseline Interferometry ( VLBI ) .
On the other hand , the Gaia mission is providing the largest , uniform sample of parallaxes for O-type stars in the entire Milky Way .
Based upon the VLBI maser and Gaia O-star parallax measurements , nearby spiral structures of the Perseus , Local , Sagittarius and Scutum Arms are determined in unprecedented detail .
Meanwhile , we estimate fundamental Galactic parameters of the distance to the Galactic center , R _ { 0 } , to be 8.35 \pm 0.18 kpc , and circular rotation speed at the Sun , \Theta _ { 0 } , to be 240 \pm 10 km s ^ { -1 } .
We found kinematic differences between O stars and interstellar masers : the O stars , on average , rotate faster , > 8 km s ^ { -1 } than maser-traced high-mass star forming regions .
alaxy : structure — Galaxy : kinematics and dynamics — masers — techniques : high angular resolution — astrometry — stars : formation