\subsection{Data Editing}
In this work we analyze two versions of our full dataset, pruned differently. 

The first version includes every pixel that is covered in every data set that we use. No other editing is done for the first version. Since the coverage of the molecular line maps is the smallest among the observations described in the previous section, the constraint is mainly set by the boundary of the molecular line maps.

In the second version, we follow Pineda et al (2008) and edit out the pixels that are less reliable. We first require that the integrated $^{12}$CO (1-0) and $^{13}$CO (1-0) emission is positive, and the main beam temperature at the peak of each line is larger than 10 and 5 times the rms noise of the $^{12}$CO and $^{13}$CO line profile, respectively. We then assume that the $^{12}$CO line is broader kinematically than the $^{13}$CO line and exclude pixels with a line width of $^{12}$CO (1-0) smaller than 0.8 times the linewidth of $^{13}$CO (1-0). Lastly, to address the problem that evolved embedded YSOs can be confused with background stars, we exclude pixels with a stellar density larger than 10. Following the second set of criteria, we obtain a map that has 10825 pixels. This is 4276 pixels fewer than the map we get from the first set of data editing.

To make maps that can be compared to each other, we consistently convolve all maps to a resolution of 3', which is the resolution of the 2MASS/NICER extinction. The convolution is done using a two dimensional Gaussian beam with FWHM = 3'. Then the maps are regridded using the Montage package (by IPAC) to match the 1.5'-pixels on the 2MASS/NICER extinction map. 