Departure of the spine with respect to the apparent midplane (defined in Sect. 3.2 as the plane at 29° PA) as measured on the PCA-disc image. NE is to the left. The uncertainty, shown here in red, is the 3σ error in the centroid position of the Lorentzian profile fitted to each vertical profile, after setting a radially-dependent noise level to each pixel. The waviness of the curve is due to the background and remaining speckle noise, as shown in Sect. 5.2. We note, however, a larger ripple at 0.8″ on the NE side.