Infrared dark clouds (IRDCs) are seen in absorption against the mid-infrared back-ground and are thought to represent likely sites of future massive star formation. We investigate these IRDCs to probe the conditions present when star formation begins. The Spitzer dark cloud (SDC) catalogue contains ∼11,000 IRDCs. We extend this catalogue to include the inner 20◦ of the Galactic plane, adding 4334 SDCs to the catalogue. Some of the objects in the catalogue are artefacts - 'dips' in the mid-infrared emission rather than regions where the emission is absorbed. With the advent of data from the Herschel satellite, we are able to construct column density maps of the objects in the SDC catalogue to identify which SDCs are true IRDCs and which are artefacts. We compare the properties of the IRDCs in the Galactic centre and in star forming regions with those in more quiescent regions. We find that the IRDCs towards star forming regions and the Galactic centre tend to have a higher column density and a slightly higher temperature, implying that the conditions within IRDCs are dependent on the environment in which they are found.Star formation has long been associated with filaments. Filaments containing longstrings of IRDCs have been observed in the Galactic plane. We apply a minimum spanning tree (MST) algorithm to the SDC catalogue to identify 88 filamentary candidates, 22 of which appear to be isolated, linear filaments similar to the Nessie nebula. Filaments tend to fragment into clumps regularly spaced along the length of the filaments. We compare theoretical predictions of this fragmentation with the clumps observed in the 22 linear filaments identified by the MST algorithm and find our results are consistent with those predicted by the sausage instability for filaments dominated by thermal pressure.