  We present a revision to the astrometric calibration of the Gemini Planet
Imager (GPI), an instrument designed to achieve the high contrast at small
angular separations necessary to image substellar and planetary-mass companions
around nearby, young stars. We identified several issues with the GPI Data
Reduction Pipeline (DRP) that significantly affected the determination of angle
of north in reduced GPI images. As well as introducing a small error in
position angle measurements for targets observed at small zenith distances,
this error led to a significant error in the previous astrometric calibration
that has affected all subsequent astrometric measurements. We present a
detailed description of these issues, and how they were corrected. We reduced
GPI observations of calibration binaries taken periodically since the
instrument was commissioned in 2014 using an updated version of the DRP. These
measurements were compared to observations obtained with the NIRC2 instrument
on Keck II, an instrument with an excellent astrometric calibration, allowing
us to derive an updated plate scale and north offset angle for GPI. This
revised astrometric calibration should be used to calibrate all measurements
obtained with GPI for the purposes of precision astrometry.
