The specific quote from VLBA page that states that the parallaxes are there being measured "to determine the complete 3D structure of the Milky Way": https://s

The specific quote from VLBA page that states that the parallaxes are there being measured "to determine the complete 3D structure of the Milky Way": https://science.nrao.edu/facilities/vlba https://science.nrao.edu/facilities/vlba "In 2010, the VLBA will begin a long-term program to determine the complete 3D structure of the Milky Way by measuring parallaxes with 10 mas accuracy or better to ~ 400 high-mass star-formation regions." In order to achieve the stated accuracy using the radio telescopes around the world together is necessary. http://www.vlba.nrao.edu/sites/ http://www.vlba.nrao.edu/sites/