Correlations between method B column density predictions (left frames) and method E column density predictions (right frames) with the values derived from noiseless data with a uniform resolution of 18.3″. The model consists of compact sources with radially increasing or decreasing temperature. The noise levels k_n are indicated in the frames. The colours indicate the pixels in the warm sources (15–20 K, red points) and the cold sources (10–15 K, blue points).