In recent years it has become clear that the Milky Way is an important testing ground for theories of galaxy formation. Much of this growth has been driven by large surveys, both photometric and spectroscopic, which are producing vast and rich catalogs of data. Through the analysis of these data sets we can gain new and detailed insights into the physical processes which shaped the Milky Way's evolution. This review will discuss a number of these developments, first focusing on the disk of the Milky Way, and then looking at its satellite population. The importance of surveys has not gone unnoticed by the Chinese astronomy community and in the final section we discuss a number of Chinese projects that are set to play a key role in the development of this field.

We investigate the correlation between extinction and H i and CO emission at intermediate and high Galactic latitudes (|b| ą 10˝) within the footprint of the Xuyi Schmidt Telescope Photometric Survey of the Galactic anticentre (XSTPS-GAC) on small and large scales. In Paper I (Chen et al. 2014), we present a three-dimensional dust extinction map within the footprint of XSTPS-GAC, covering a sky area of over 6,000 deg 2 at a spatial angular resolution of 6 arcmin. In the current work, the map is combined with data from gas tracers, including H i data from the Galactic Arecibo L-band Feed Array H i survey and CO data from the Planck mission, to constrain the values of dust-to-gas ratio DGR " A V {NpHq and CO-to-H 2 conversion factor X CO " NpH 2 q{W CO for the entire GAC footprint excluding the Galactic plane, as well as for selected star-forming regions (such as the Orion, Taurus and Perseus clouds) and a region of diffuse gas in the northern Galactic hemisphere. For the whole GAC footprint, we find DGR " p4.15˘0.01qˆ10´2 2 mag cm 2 and X CO " p1.72˘0.03qˆ10 20 cm´2 pK km s´1q´1. We have also investigated the distribution of "CO-dark" gas (DG) within the footprint of GAC and found a linear correlation between the DG column density and the V-band extinction: NpDGq » 2.2ˆ10 21 pA V´A c V q cm´2. The mass fraction of DG is found to be f DG " 0.55 toward the Galactic anticentre, which is respectively about 23 and 124 per cent of the atomic and CO-traced molecular gas in the same region. This result is consistent with the theoretical work of Papadopoulos et al. but much larger than that expected in the H 2 cloud models by Wolfire et al.