Schematic representation of ensembles of shocks and PDRs inside an observational beam with a solid angle Ω_obs (black and white circle). Shock or PDR surfaces are represented by the filled circles and filled squares, respectively, and are randomly placed within the beam. The color represents the systemic velocities of shock and PDR surfaces projected along the line of sight. Blue (resp. red) circles and squares correspond to blueshifted (resp. redshifted) shocks and PDRs with respect to the recession velocity of the galaxy. The left and right panels represent two equivalent scenarios, where the numbers of shock (resp. PDR) surfaces are different, but subtend the same total solid Ω_S (resp. Ω_P). This figure is adapted from Lehmann & Godard (2022).