Variations of the square root of the Cramer-Rao bound (CRB) of the centroid velocity (Δ_V, left panel) and velocity dispersion (σ_V, middle panel) in km s⁻¹ as afunction of the column density and the excitation temperature. Right panel: variations of a function of the product of the number of channels (K) and the signal-to-noise ratio . In all three cases, the data are simulated assuming that ¹³CO(1−0) and ¹³CO(2− 1) are measured and the unit of the image contours are km s⁻¹. In this simulation, σ_b = 100 mK, Δ_V = 1.1 km s⁻¹ and σ_V = 0.61 km s⁻¹.