2D histograms of velocity dispersion and estimated vertical distances z_gal for the BDC run with (left) and without (right) the spiral arm prior. The blue line shows the median σ_v value per z_gal bin. The small strips at the top of the individual panels show where the median value is higher (blue) or lower (red) compared to the opposite BDC run, with the strength of the colour corresponding to the magnitude of the difference. The dashed horizontal line indicates the GRS velocity resolution (0.21 km s⁻¹).