via the <span style="font-variant:small-caps;">astroquery</span> module. The WCS-corrected images are then merged using the <span style="font-variant:small-caps;">astrodrizzle</span> routine, with a pixel scale set to 0.04arcsec for both filters (for simplicity).

![image](fig3.pdf){width="\textwidth"}

Spectral Extraction
-------------------

The spectra of 20 prominent shocked ISM clouds were extracted from the global WiFeS mosaic datacubes using using [QFitsView v3.1 rev.741]{}[^3]. The positions of the clouds were chosen so as to largely avoid contamination by high-velocity \[

\] – emitting material, [ see Fig \[fig2\]]{}. The exception is P07, but the velocity shift of the high velocity material here is sufficient as to allow accurate determinations of the narrow-line fluxes. We used a circular extraction aperture with a radius of either 2 or 3 arc sec. to best match the size of the bright region. To remove the residual night sky emission and (approximately) the faint stellar contribution, we subtracted a mean sky reference annulus 1 arc sec. wide surrounding the extraction aperture. The extraction regions were optimised by peaking up the signal in H$\alpha$ in the red data cube, and in H$\gamma$ in the blue data cube, respectively. This procedure may lead to some contamination by the faint extended cloud emission in the case of P07, P08, 