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Speed and Accuracy Considerations with Optimization

Updated August 02, 2016

This article applies to:
  • Softree Optimal

Optimization can take a long time.It is not difficult to generate problems which take hours (or even days -- yes really!). Unfortunately, there is no way to know in advance how long a specific problem will take.Here are some recommendations:

  • In the Options Standards tab, start by using Curves [Fast] (or Polyline). This will generate the lowest cost alignment possible for a given set of constraints; use this cost and the profile as your baseline for evaluating future alignments. If satisfied with the alignment and a solution has been found, create a New alignment then switch to Curves and tangents [Slow] or Variables curves and tangents [Slowest]. Note these two algorithms will take longer to solve.
  • For accuracy purposes, the user should set the Spacing to the recommended Auto calculated spacing (see Section Sampling and Accurate Costing). Such a choice might yield a very large number of sections. In that case, you have two options:
    • Divide your alignment into pieces; see Optimizing Long Roads.
    • Clear the All check box in Sections Sampled and possibly increase the Spacing. This will speed thing up but reduce accuracy.

Figure 1 Section Sampling Option

  • Once the alignment is being optimized (after Pre-Process), a dialogue box appears displaying the Optimality gap. The Optimality gap is defined as the best improvement that can be made over the current solution. (Note that this is an upper bound; in fact, the current solution may already be the optimal solution.) Rather than wait for the optimality, when the optimality gap reaches zero, you may cancel at any time to accept the current best solution. So, if you cancel the process at 5%, the cost of the alignment obtained will be within 5% of optimal cost. Often, the time it takes to find a solution with a small Optimality gap is much less than the time to reach optimality.