Example of HI spectra towards HII regions in the first and fourth quadrant extracted from 21 cm line cubes from the VGPS and SGPS. The source velocity is illustrated as solid red vertical line, the tangent point velocity as solid blue vertical line, and the velocity, at which the absorption is below 5σ, as dashed-dotted green vertical line. This limit of −5σ is indicated by the solid black horizontal line. In addition, the maximum positive intensity of HII regions in the first quadrant must be lower than 5σ, which is shown as dashed black horizontal line. The dashed black vertical lines ±8.28 km s^-1 around the source velocity display its variation because of the velocity dispersion of clouds and streaming motions (see Sect. 5.4). Examples of sources in the first quadrant are shown on the left and of clumps in the fourth quadrant on the right. a) There is a large difference between the tangent point velocity and the velocity, up to which HI absorption is seen and which is close to the source velocity for G8.14+0.23 and G337.92−0.46, which are thus located at the near distance. They are located in the diagonal right region in Fig. 4 (red points) confined by the amount of streaming motions. b) Our analysis reveals HI absorption at velocities close to the tangent point velocity for G37.37−0.23 and G337.12−0.18, which are assigned to the far distance and indicated as blue points in the horizontal right part in Fig. 4. c) We locate G49.37−0.30 and G309.89+0.40 at the tangent point because the source velocity is within 8.28 km s^-1 around the tangent point velocity. They are displayed as purple points in Fig. 4.