  Obtaining pulsar parallaxes via relative astrometry (also known as
differential astrometry) yields distances and transverse velocities that can be
used to probe properties of the pulsar population and the interstellar medium.
Large programs are essential to obtain the sample sizes necessary for these
population studies, but they must be efficiently conducted to avoid requiring
an infeasible amount of observing time. This paper describes the PSRPI
astrometric program, including the use of new features in the DiFX software
correlator to efficiently locate calibrator sources, selection and observing
strategies for a sample of 60 pulsars, initial results, and likely science
outcomes. Potential applications of high-precision relative astrometry to
measure source structure evolution in defining sources of the International
Celestial Referent Frame are also discussed.
