Mapping the Galactic Metallicity Gradient with Open Clusters: The
  State-of-the-Art and Future Challenges
  In this paper, we make use of data collected for open cluster members by
high-resolution spectroscopic surveys and programmes (i.e., APOGEE, Gaia-ESO,
GALAH, OCCASO, and SPA). These data have been homogenised and then analysed as
a whole. The resulting catalogue contains [Fe/H] and orbital parameters for 251
Galactic open clusters. The slope of the radial metallicity gradient obtained
through 175 open clusters with high-quality metallicity determinations is
$-$0.064 $\pm$ 0.007 dex kpc$^{-1}$. The radial metallicity distribution traced
by open clusters flattens beyond R$_{\rm Gal}$=12.1 $\pm$ 1.1 kpc. The slope
traced by open clusters in the [Fe/H]-L$_{\rm z}$ diagram is $-$0.31 $\pm$ 0.02
10$^{3}$ dex km$^{-1}$ kpc$^{-1}$ s, but it flattens beyond L$_{\rm z}$=2769
$\pm$ 177 km kpc s$^{-1}$. In this paper, we also review some high-priority
practical challenges around the study of open clusters that will significantly
push our understanding beyond the state-of-the-art. Finally, we compare the
shape of the galactic radial metallicity gradient to those of other spiral
galaxies.
