Spatial distribution of the dust-traced H₂ column density derived from Herschel data (André et al. 2010; Lombardi et al. 2014). The dotted grid is used to define the training and test sets. The white square in the bottom right around the Horsehead region corresponds to the test set where the random forest predictions will be compared to the observations. This subset is never used during the training phase. Only the remainder of the map is used as the training set.