Astrometric Very Long Baseline Interferometry ( VLBI ) observations of maser sources in the Milky Way are used to map the spiral structure of our Galaxy and to determine fundamental parameters such as the rotation velocity ( \Theta _ { 0 } ) and curve and the distance to the Galactic center ( R _ { 0 } ) .
Here , we present an update on our first results , implementing a recent change in the knowledge about the Solar motion .
It seems unavoidable that the IAU recommended values for R _ { 0 } and \Theta _ { 0 } need a substantial revision .
In particular the combination of 8.5 kpc and 220 km s ^ { -1 } can be ruled out with high confidence .
Combining the maser data with the distance to the Galactic center from stellar orbits and the proper motion of Sgr A* gives best values of R _ { 0 } = 8.3 \pm 0.23 kpc and \Theta _ { 0 } = 239 or 246 \pm 7 km s ^ { -1 } , for Solar motions of V _ { \odot } = 12.23 and 5.25 km s ^ { -1 } , respectively .
Finally , we give an outlook to future observations in the Bar and Spiral Structure Legacy ( BeSSeL ) Survey .