Comparison between the V_LSR of C¹⁸O and SO (blue squares), and C¹⁸O and N₂H⁺ (red triangles) in L1517 illustrating the good agreement between all tracers. The central dashed line indicates the locus of equal velocities, and the surrounding dotted lines have been displaced by the sound speed (0.19 km s^-1 for ISM gas at 10 K). The mean formal error in the V_LSR determination is 0.02 km s^-1 for SO and 0.01 km s^-1 for both C¹⁸O and N₂H⁺, which is comparable or smaller than the marker size.