Spiral arm structure near tangent points. The figure is arranged in four quadrants with each containing paired images showing longitude-distance relationships (top) and corresponding sky distributions (bottom). Top left: Scutum and Norma arms (first quadrant). Top right: Crux and Norma arms (fourth quadrant). Bottom left: Sagittarius arm. Bottom right: Carina arm. Spiral arm fits (Sect. 4.2.2) are shown as dotted lines, with maser positions and their error bars colour-coded by arm: Norma (magenta), Scutum/Crux (red), and Sagittarius/Carina (green)