  We describe the astrometric calibration of the Gemini NICI Planet-Finding
Campaign. The Campaign requires a relative astrometric accuracy of $\approx$ 20
mas across multi-year timescales in order to distinguish true companions from
background stars by verifying common proper motion and parallax with their
parent stars. The calibration consists of a correction for instrumental optical
image distortion, plus on-sky imaging of astrometric fields to determine the
pixel scale and image orientation. We achieve an accuracy of $\lesssim 7$ mas
between the center and edge of the 18$''$ NICI field, meeting the 20 mas
requirement. Most of the Campaign data in the Gemini Science Archive are
accurate to this level but we identify a number of anomalies and present
methods to correct the errors.
