Relations between the column densities of H₂ derived from CO and dust. Upper: correlation between the column densities of H₂ derived from¹³CO (N^13CO(H₂)) and C¹⁸O (N^C18O(H₂)) at the positions of the FAST beams for 11 PGCCs (G174.08-13.2 is not included because of lacking CO mapping observations). Data of different sources are shown in different colors. Lower: correlation between the H₂ column densities at the map centers derived from dust emissions (N^dust(H₂)) and C¹⁸O (N^C18O(H₂)).