Observed major axis position-velocity diagram of IC 2531 (black and grey contours) overlaid with different models. The green contours represent the final (W) model. Other models are shown as red contours. Contour levels are −1.6, 1.6 (1.5σ), 4.9, and 8.1 mJy beam^-1 and the greyscale corresponds to the observations. The high-density ridge along the low-velocity edge on both sides (indicated by the arrows) is not reproduced by the W model.