Comparison of baseline fits toward continuum emission. The black curve shows an example spectrum of H I emission centered in ℓ = 18.95°, b = − 0.03° that is contaminated by continuum emission. The dashed magenta and green curve show a second and first order polynomial fit to the velocity channels of the H I spectrum at 33.5–43.0 and 56.0–65.5 km s⁻¹, respectively.Due to the continuum contamination, the second order polynomial yields a bad fit to the HISA baseline.