CO line ratios in our fields based on Planck Type 1 and Type 2 CO maps. Type 1 ratios (2–1)/(1–0) and (3–2)/(1–0) are plotted with blue and red circles and Type 2 ratios (2–1)/(1–0) with green squares. The open and the filled symbols are averages over pixels below and above median column density, respectively. The black lines show least squares fits to data including both low and high column densities. The shaded areas indicate the range of line ratios considered in this paper.