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We present a revision to the astrometric calibration of the Gemini PlanetImager (GPI), an instrument designed to achieve the high contrast at smallangular separations necessary to image substellar and planetary-mass companionsaround nearby, young stars. We identified several issues with the GPI DataReduction Pipeline (DRP) that significantly affected the determination of angleof north in reduced GPI images. As well as introducing a small error inposition angle measurements for targets observed at small zenith distances,this error led to a significant error in the previous astrometric calibrationthat has affected all subsequent astrometric measurements. We present adetailed description of these issues, and how they were corrected. We reducedGPI observations of calibration binaries taken periodically since theinstrument was commissioned in 2014 using an updated version of the DRP. Thesemeasurements were compared to observations obtained with the NIRC2 instrumenton Keck II, an instrument with an excellent astrometric calibration, allowingus to derive an updated plate scale and north offset angle for GPI. Thisrevised astrometric calibration should be used to calibrate all measurementsobtained with GPI for the purposes of precision astrometry.

Revised Astrometric Calibration of the Gemini Planet Imager