CO polar image of NGC 4321 with the bar lane masks on top. The center is masked out with an environmental mask, as well as the spiral arms. Emission at radii larger than the full bar length (yellow horizontal line) is also masked out (see Table D.1). The selected azimuthal range for each bar lane (first bar lane ID 1: Δθ_bl1 blue dashed line, second bar lane ID 2: Δθ_bl2 red dashed line) used for the fitting is indicated. The bar lane mask area is defined by two shifted exponential profiles is shown in colorscale, the area outside in greyscale. The starting angle corresponds to a position angle of 90 degrees (measured clockwise from north) in the deprojected map, further environmental region masks are applied.