Distribution of column density estimates in units of equivalent V-band extinction, based on tracers used in this paper. The blue curve in each plot is the normal profile fitted to the distribution of 2MASS/NICER extinction, shown in the first plot. Curves in subsequent plots, coded in red and green, are the normal profiles fitted to distributions shown in these plots. The standard deviation $\sigma$ for the normal profiles are represented by the horizontal bars in the first three plots. In the last plot, the green histogram shows the distribution of the column density estimate based on N($^{13}$CO), and the dark gray filled histogram shows the distribution based on the integrated $^{13}$CO (1-0) line emission. See \S3.3 for details about the fitting methods.