The second Herschel-ATLAS Data Release - III: optical and near-infrared
  counterparts in the North Galactic Plane field
  This paper forms part of the second major public data release of the Herschel
Astrophysical Terahertz Large Area Survey (H-ATLAS). In this work, we describe
the identification of optical and near-infrared counterparts to the
submillimetre detected sources in the $177$ deg$^2$ North Galactic Plane (NGP)
field. We used the likelihood ratio method to identify counterparts in the
Sloan Digital Sky Survey and in the UKIRT Imaging Deep Sky Survey within a
search radius of $10$ arcsec of the H-ATLAS sources with a $4\sigma$ detection
at $250 \, \mu$m. We obtained reliable ($R \ge 0.8 $) optical counterparts with
$r< 22.4$ for 42429 H-ATLAS sources ($37.8$ per cent), with an estimated
completeness of $71.7$ per cent and a false identification rate of $4.7$ per
cent. We also identified counterparts in the near-infrared using deeper
$K$-band data which covers a smaller $\sim25$ deg$^2$. We found reliable
near-infrared counterparts to $61.8$ per cent of the $250$-$\mu$m-selected
sources within that area. We assessed the performance of the likelihood ratio
method to identify optical and near-infrared counterparts taking into account
the depth and area of both input catalogues. Using catalogues with the same
surface density of objects in the overlapping $\sim25$ deg$^2$ area, we
obtained that the reliable fraction in the near-infrared ($54.8$ per cent) is
significantly higher than in the optical ($36.4$ per cent). Finally, using deep
radio data which covers a small region of the NGP field, we found that $80 -
90$ per cent of our reliable identifications are correct.
