Mapping the Milky Way and the Local Group.

Over the past decade, the astrometric accuracy of Very Long Baseline Inteferometry has improved dramatically. Currently relative positions between sources separated by about 1° are being measured with accuracies of ~ 10 μas. With this accuracy, trigonometric parallaxes throughout the Milky Way and proper motions of Local Group galaxies are now being measured. These observations will lead to direct mapping of the structure of the Milky Way, testing of the spiral density wave paradigm, measuring the dark matter halos of Local Group galaxies, and better understanding of the past history and future fate of these galaxies. [ABSTRACT FROM AUTHOR]