Surface density (kpc−2) in the Galactic disk of APOGEE stars in our subsample used in this paper. The label "GC" indicates the Galactic Center, and the circled dot is at the position of the Sun (assuming RGC, ☉ = 8 kpc). The dotted circles have constant Galactocentric distances of RGC = 4, 8, 12, 16, and 20 kpc.