  Galactic plane surveys of pristine molecular clouds are key for establishing
a Galactic-scale view of the earliest stages of star formation. For this reason
Peretto & Fuller (2009) built an unbiased sample of IRDCs in the 10 deg < |l| <
65 deg, |b|<1 deg region of the Galactic plane using Spitzer 8micron
extinction. However, in absorption studies, intrinsic fluctuations in the
mid-infrared background can be mis-interpreted as foreground clouds. The main
goal of the study presented here is to disentangle real clouds in the Spitzer
Dark Cloud (SDC) catalogue from artefacts due to fluctuations in the
mid-infrared background. We constructed H_2 column density maps at ~18"
resolution using the 160micron and 250micron data from the Herschel Galactic
plane survey Hi-GAL. We also developed an automated detection scheme that
confirms the existence of a SDC through its association with a peak on these
Herschel column density maps. Detection simulations, along with visual
inspection of a small sub-sample of SDCs, have been performed to get better
insight into the limitations of our automated identification scheme. Our
analysis shows that 76(+/-19)% of the catalogued SDCs are real. This fraction
drops to 55(+/-12)% for clouds with angular diameters larger than ~1 arcminute.
The contamination of the PF09 catalogue by large spurious sources reflect the
large uncertainties associated to the construction of the 8micron background
emission, a key stage towards the identification of SDCs. A comparison of the
Herschel confirmed SDC sample with the BGPS and ATLASGAL samples shows that
SDCs probe a unique range of cloud properties, reaching down to more compact
and lower column density clouds than any of these two (sub-)millimetre Galactic
plane surveys.
