Top panel: expected locations of the OCs we failed to detect in this study (excluding infrared clusters, dubious clusters and proposed asterisms). S14S15 indicates the candidates of Schmeja et al. (2014) and Scholz et al. (2015), and FSR those of Froebrich et al. (2007). Bottom left panel: distribution in Galactocentric distance of the identified OCs (green) and those we failed to identify, excluding the candidates of S14S15 and FSR (dashed black). The vertical dotted line indicates the solar radius at 8.34 kpc. Bottom right panel: same as previous panel, for the distribution in heliocentric distance.