Longitude-velocity diagrams for the HII regions (upper), GMCs (middle), and 6.7 GHz methanol masers (lower), which are overlaid on the image of ¹²CO (1−0) of Dame et al. (2001, the IAU standard solar motions are adopted). The open squares indicate HII regions with the symbol area proportional to the excitation parameters (see Eq. (1)). The open circles indicate the GMCs with the symbol size proportional to log  (M_GMCs) (see Sect. 2.4). The black crosses indicate the 6.7 GHz methanol masers with a constant symbol size. A spiral arm model (see the left panel of Fig. 11) is overlaid to indicate the locations of the identified arm segments: arm-1: the Norma Arm; arm-2: the Scutum-Centaurus Arm; arm-3: the Sagittarius-Carina Arm; arm-4: the Perseus Arm; arm-5: the Outer Arm; arm-6: the Outer+1 Arm; and the Local Arm is also given.