mask 2 is the densest portion of the cloud, we assume that the  levels are populated in LTE at the kinetic temperature, but we calculate a nominal value for the optical depth from the ratio of the peak  and  intensities using the usual equation of radiative transfer in a uniform medium. We assume a  to  abundance ratio of 65, very close to the average value for local clouds found by [@langer1993]. We use the value of optical depth obtained to make a saturation correction to the  column density derived assuming optically thin emission, with the usual formula N(\^[13]{}[CO corrected]{}) = N(\^[13]{}[CO assumed optically thin)]{}

Mask 1 presents the greatest challenge in terms of column density determination since it encompasses approximately one third of the area mapped and has reasonably strong  emission. However, since the  is not detected in individual pixels, we need a different scheme to extract the column density. We have developed a statistical approach, which should be applicable to other large maps in which only the more abundant isotopologue is detected in individual pixels. The procedure assumes that the  is optically thick at its peak, and that the value of the antenna temperature can directly be converted to 