IGAPS: the merged IPHAS and UVEX optical surveys of theNorthern Galactic
  Plane
  The INT Galactic Plane Survey (IGAPS) is the merger of the optical
photometric surveys, IPHAS and UVEX, based on data from the Isaac Newton
Telescope (INT) obtained between 2003 and 2018. Here, we present the IGAPS
point source catalogue. It contains 295.4 million rows providing photometry in
the filters, i, r, narrow-band Halpha, g and U_RGO. The IGAPS footprint fills
the Galactic coordinate range, |b| < 5deg and 30deg < l < 215deg. A uniform
calibration, referred to the Pan-STARRS system, is applied to g, r and i, while
the Halpha calibration is linked to r and then is reconciled via field
overlaps. The astrometry in all 5 bands has been recalculated on the Gaia DR2
frame. Down to i ~ 20 mag (Vega system), most stars are also detected in g, r
and Halpha. As exposures in the r band were obtained within the IPHAS and UVEX
surveys a few years apart, typically, the catalogue includes two distinct r
measures, r_I and r_U. The r 10sigma limiting magnitude is ~21, with median
seeing 1.1 arcsec. Between ~13th and ~19th magnitudes in all bands, the
photometry is internally reproducible to within 0.02 magnitudes. Stars brighter
than r=19.5 have been tested for narrow-band Halpha excess signalling line
emission, and for variation exceeding |r_I-r_U| = 0.2 mag. We find and flag
8292 candidate emission line stars and over 53000 variables (both at >5sigma
confidence). The 174-column catalogue will be available via CDS Strasbourg.
