PTV Visum - Frequently Asked Questions (FAQs)

(#VIS24174)

Running TFlowFuzzy leads to a warning:
'An error occurred during the calculation of the new demand matrix. The corrected demand for at least one relation is not within the valid value range. Please check the demand of the relations listed in the log file with a very large correction factor. If this demand is very small (approximately < 10^-5), it should be set to 0 and, if necessary, the demand matrix should be scaled in such a way that the total demand is retained.'

Some calculated volumes and count values differ too much, while the correction range is too low.
Please note that TFlowFuzzy is a mathematical procedure to correct an already nearly matching demand matrix. It was never intended to enable the estimation of a demand matrix starting from a dummy matrix.

Check the following:
- The error message might point to probably rather not relevant small demand values, which receive a proportionally rather high corrected value. Recommendation: Initialize values < 0.00001 to 0.
- Start with higher bandwidths (tolerance values), and if a solution is found lower them again.
- Choose a lower max. correction factor.
- Choose a higher alpha level to include a wider range of values into the fuzzy set. In general this approach is recommended: start with a higher alpha level, run TFlowFuzzy and reduce alpha to lower values step by step.
- Take only a few counts and demand entries first, preferably the higher numbers. If this already fails, you need to look for an error in this group, thus reduce the number of entries and counts again. Once you succeed, you can add more and more counts and demand entries. Remember TFlowFuzzy can help you to correct an already, nearly correct demand matrix, based upon counted values. Take care that the few largest counted values you are using or adding do make sense and match to your demand matrix.
- Check for contradictory counted values. Leave those values out which seem to be unrealistic or cannot be reached.
- Check the reasonability of the paths found by the assignment. Take care of its parameters and number of iterations.
- Use the analysis tools Protocol, Diagnosis and Process statistics (see the Online Help for further instructions, in the dialog press F1): TFlowFuzzy -> Parameters.

Use the example and tutorial files:
Help -> Examples -> Open Examples directory
1) folder c:\Users\Public\Documents\PTV Vision\PTV Visum 18\Examples\Matrix TFlowFuzzy\
2) folder c:\Users\Public\Documents\PTV Vision\PTV Visum 18\Tutorials\TFlowFuzzy\

Refer also to:
http://vision-traffic.ptvgroup.com/en-uk/training-support/support/ptv-visum/faqs/visfaq/show/VIS12676/