Radial variation of the electron densities (n_e([O II]) in dark blue circles and n_e([S II]) in light blue squares) as a function of the distance to the ionizing star HD 164492A. Faint points represent individual spaxels, while opaque points show the uncertainty-weighted average of all spaxels within 0.1 pc wide distance bins. Standard deviations are shown as error bars. The extent of the measured Strömgren sphere (Sect. 3.4) is shown by a grey dashed line together with the error as a grey shaded region.