Assessment of [Fe/H] determinations for FGK stars in spectroscopic
  surveys
  The number of stars with a measured value of [Fe/H] is considerably
increasing thanks to spectroscopic surveys. However different methodologies,
inputs and assumptions used in spectral analyses lead to different precisions
in [Fe/H] and possibly to systematic differences that need to be evaluated. It
is essential to understand the characteristics of each survey to fully exploit
their potential, in particular if the surveys are combined. The purpose of this
study is to compare [Fe/H] determinations from the largest spectroscopic
surveys (APOGEE, GALAH, the Gaia ESO survey, RAVE, LAMOST, SEGUE ) to other
catalogues taken as reference. Offsets and dispersions of the residuals are
examined as well as their trends with other parameters. We use reference
samples providing independent determinations of [Fe/H] which are compared to
those from the surveys for common stars. The distribution of the residuals is
assessed through simple statistics that measures the offset between two
catalogues and the dispersion representative of the precision of both
catalogues. When relevant, linear fits are performed. A large sample of
FGK-type stars with [Fe/H] based on high-resolution, high signal to noise
spectroscopy was built from the PASTEL catalogue to provide a reference sample.
We also use FGK members in open and globular clusters to assess the internal
consistency of [Fe/H] of each survey. The agreement of median [Fe/H] values for
clusters observed by different surveys is discussed. All the surveys
overestimate the low metallicities, and some of them also underestimate the
high metallicities. They perform well in the most populated intermediate
metallicity range, whatever the resolution. In most cases the typical precision
that we deduce from the comparisons is in good agreement with the uncertainties
quoted in the catalogues. Some exceptions to this general behaviour are
discussed.
