The Gaia/IPHAS and Gaia/KIS Value-Added Catalogues
  We present a sub-arcsecond cross-match of Gaia DR2 against the INT
Photometric H-alpha Survey of the Northern Galactic Plane Data Release 2 (IPHAS
DR2) and the Kepler-INT Survey (KIS). The resulting value-added catalogues
(VACs) provide additional precise photometry to the Gaia photometry (r, i and
H-alpha for IPHAS, with additional U and g for KIS). In building the catalogue,
proper motions given in \gaia\ DR2 are wound back to match the epochs of IPHAS
DR2, thus ensuring high proper motion objects are appropriately cross-matched.
The catalogues contain 7,927,224 and 791,071 sources for IPHAS and KIS,
respectively. The requirement of >5-sigma parallax detection for every included
source means that distances out to 1--1.5 kpc are well covered. We define two
additional parameters for each catalogued object: (i) $f_c$, a
magnitude-dependent tracer of the quality of the Gaia astrometric fit; (ii)
$f_{FP}$, the false-positive rate for parallax measurements determined from
astrometric fits of a given quality at a given magnitude. Selection cuts based
on these parameters can be used to clean colour-magnitude and colour-colour
diagrams in a controlled and justified manner. We provide both full and light
versions of the VAC, with VAC-light containing only objects that represent our
recommended trade-off between purity and completeness. Uses of the catalogues
include the identification of new variable stars in the matched data sets, and
more complete identification of H-alpha-excess emission objects thanks to
separation of high-luminosity stars from the main sequence.
