  We present a new three-dimensional map of dust reddening, based on Gaia
parallaxes and stellar photometry from Pan-STARRS 1 and 2MASS. This map covers
the sky north of a declination of -30 degrees, out to a distance of several
kiloparsecs. This new map contains three major improvements over our previous
work. First, the inclusion of Gaia parallaxes dramatically improves distance
estimates to nearby stars. Second, we incorporate a spatial prior that
correlates the dust density across nearby sightlines. This produces a smoother
map, with more isotropic clouds and smaller distance uncertainties,
particularly to clouds within the nearest kiloparsec. Third, we infer the dust
density with a distance resolution that is four times finer than in our
previous work, to accommodate the improvements in signal-to-noise enabled by
the other improvements. As part of this work, we infer the distances,
reddenings and types of 799 million stars. We obtain typical reddening
uncertainties that are ~30% smaller than those reported in the Gaia DR2
catalog, reflecting the greater number of photometric passbands that enter into
our analysis. Our 3D dust map can be accessed at
https://doi.org/10.7910/DVN/2EJ9TX or through the Python package "dustmaps,"
and can be queried interactively at http://argonaut.skymaps.info. Our catalog
of stellar parameters can be accessed at https://doi.org/10.7910/DVN/AV9GXO.
