| Differential astrometry using Very Long Baseline Interferometry allows extremely precise measurement of the distance and transverse velocity of compact objects in the Galaxy. Large projects are currently underway to map the Galactic distributions of high mass star forming regions, low mass star forming regions, and pulsars, which will together give us an unprecedented view of the ionised gas, neutral gas, dust, and stars in the Milky Way. I will describe the PSRPI project currently underway on the VLBA, which has the ultimate goal of measuring 200 pulsar distances (10% of the pulsar population) in an accurate and model-independent manner. This information will allow us to greatly improve models of the ionized gas distribution in the Galaxy whilst simultaneously removing the existing biases in estimates of the pulsar velocity and luminosity distribution. To make such a large project feasible, a number of advances in VLBI techniques were necessary; I will describe these advances and present some early results, as well as the prospects for continued improvement in astrometric accuracy with future facilities such as the SKA.
Image caption: an image from the FIRST radio survey showing the area around PSR J1022+1001. The primary beam of a VLBA dish at 1.6 GHz is shown in red, and VLBI images of the pulsar (highlighted in blue) and several "in-beam" reference sources (highlighted in magenta) are overlaid. The zoom between the FIRST image and the VLBI images is around a factor of 5,000. Simultaneously observing in-beam reference sources and the target allows for extremely precise astrometry; the pulsar position can be measured to around 50 microarcsecond accuracy.