Spatial distribution of sources, colourised by their Faraday complexity classification, as described in Sect. 5.2.1. The cyan-shaded regions show longitudes that correspond to the Galactic spiral arms listed in the text and the longitude range within 5 degrees of the Galactic centre. The Faraday simple sources are plotted as black dots. The size of the markers for Faraday complex sources is scaled according to the number of QU-fitted components.