Before and after spline correction. Average of 80 spectra from an OTF scan taken as part of the [CII] Galactic center mapping project (data shown in Fig. A.3 are part of this average). Left panel: standard reduction with a third-order polynomial baseline correction. Right panel: Reduction using the spline catalog approach. We note [CII] emission from 20 to 70 km s⁻¹ and consistent line emission seen between coaligned (~2 arcseconds) H and V pixels after correction.