Example spectra of the H₂CO(3_0,3 −2_0,2) transition of target G0.253+0.016 to show the spline-fitting of bad baselines. Left: input spectrum showing a bad baseline. The red line shows a linear baseline for comparison. Middle: input spectrum with the spline spectrum (red) overlayed. Right: baseline-subtracted spectrum. The gray box shows a baseline feature that was not removed in the spline-fitting due to its narrow width.