Line stacking process for molecular lines in a spectral survey. Step A: correct observed spectrum, shown in black in the top, by the source LSR velocity (corresponding to 64 km s⁻¹ for Sgr B2(N)). Create a synthetic spectrum (shown in red, and using XCLASS) for the considered molecular species. This will beused for the identification of detectable transitions (highlighted with a gray shadow area). Step B: extract a portion of the spectrum centered at each transition, and transform the frequency axis into velocity using the Doppler equation. Each cutout spectra is uniformly resampled and rebined. Step C: sum up all the cutout spectra and divide by the number of transitions. Normalization has been waived to prevent that noise and lines from other molecular species are not scaled up. Line blendings are statistically averaged out and the true line shape and the velocity can be determined.