Takes two short drawing commands (either cubics or lines) and generates a single line between start and endpoints. Often used when an outline curve has much too much detail for what is required. Repeated uses of the shorten transformation will give a rough approximation to the original curve. Normally used in association with smooth_short_lines to smooth the shortened curve.
In the diagram below, the map outline on the left has been shortened 6 times (probably more than was sensible) followed by smoothing. It results in a much smaller file with some loss of detail.
Figure 19.1: Shorten 6 times then smooth