Scatter plot of I(¹³CO) versus I(¹²CO) for the 734 sources detected in ¹³CO. The solid line is a linear fit to the data points in the figure and indicates that I(¹²CO) is, on average, a good linear tracer of H₂ column density for I(¹²CO) > 5 K km s^-1. The 1-σ noise is indicated on two representative low intensity points; on the scale of this figure it can be seen that the signal-to-noise ratio is very high except for a few of the weakest ¹²CO points. The detection limits are given in Table 2.