Comparison of true and estimated column densities. The three columns correspond to the true column density, the column density estimated from ideal observations, and the column density derived from the nominal synthetic ngVLA observations. Results are shown for C¹⁸O (first row; optically thin approximation), combination of H¹³CO⁺ and HCO⁺ lines (second row), N₂H⁺ (third row; fit of hyperfine structure), and NH₃ (bottom row; fit of hyperfine structure). Observational estimates have been scaled to N(H₂) using the maximum fractional-abundance value in the modelling. In the first column, the true column densities are also correspondingly scaled down according to the spatial variation of the relative abundances.