Comparison of parallax distance with maximum likelihood distance in the Galactocentric coordinates frame. Left: parallax-only distances (blue). Right: maximum likelihood distances, based on parallax and 3DKD (black). The solid line is the single-segment arm fit from Table 3. With no spiral structure/arm position a priori, maser positions converge to a much cleaner spiral shape. We have excluded G021.87 and G070.33 from the parallax distance plot as they do not have a well-measured parallax. The thin concentric rings centered on the Sun (red star at [0, 8.15]) are spaced by 1 kpc, and the thick concentric circles are spaced by 5 kpc. The rays show Galactic longitude, spaced by 10 deg. The green square shows the location of the Galactic center at [0, 0] in both plots.