  The Galactic Plane Infrared Polarization Survey (GPIPS) seeks to characterize
the magnetic field in the dusty Galactic disk using near-infrared stellar
polarimetry. All GPIPS observations were completed using the 1.83 m Perkins
telescope and Mimir instrument. GPIPS observations surveyed 76 sq-deg of the
northern Galactic plane, from Galactic longitudes 18 to 56 deg and latitudes -1
to +1 deg, in the H-band (1.6 um). Surveyed stars span 7th to 16th mag,
resulting in nearly 10 million stars with measured linear polarizations. Of
these stars, ones with m_H < 12.5 mag and polarization percentage uncertainties
under 2% were judged to be high quality and number over one million. GPIPS data
reveal plane-of-sky magnetic field orientations for numerous interstellar
clouds for AV values to ~30 mag. The average sky separation of stars with m_H <
12.5 mag is about 30arcsec, or about 60 per Planck polarization resolution
element. Matching to Gaia DR2 showed the brightest GPIPS stars are red giants
with distances in the 0.6-7.5 kpc range. Polarization orientations are mostly
parallel to the Galactic disk, with some zones showing significant orientation
departures. Changes in orientations are stronger as a function of Galactic
longitude than of latitude. Considered at 10 arcmin angular scales, directions
that show the greatest polarization fractions and narrowest polarization
position angle distributions are confined to about ten large, coherent
structures that are not correlated with star forming clouds. The GPIPS
polarimetric and photometric data products (Data Release 4 catalogs and images)
are publicly available for over 13million stars.
