Left panel: proper motions of the individual features in the synthetic [S ii] map (model M2b) together with the boxes in which the cross-correlations were made. The red arrows show the computed proper motions. The length of each arrow is proportional to the proper motion. Right panel: proper motions of the individual knots of CRL 618, included for comparison. Both images are shown to the same spatial scale. The vertical arrow at the top right shows a proper motion of 300 km s^-1.