Correlations between column density estimates and the true column density. The y-axis shows the ideal and simulated ngVLA estimates, and the x-axis the true model column density of the molecule in question. Results are shown for C¹⁸O, N₂H⁺, and NH₃. These correspond to the maps in Fig. 1, with the addition of C¹⁸O estimates based on the fitting of two Gaussian velocity components (N_CASA2 in frame a). There are nine linearly spaced contour levels, up to the maximum point density (pixels per logarithmic intervals of true and observed column density).