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Section 3: GBFM Developer’s Reference
Page 49
Once the 'RunScenario' sequence has produced the base year matrices, it saves the
factors that constrain the domestic flows to the CSRGT data.
The 'RunForecastScenario' routine can be used to produce future of hypothetical trip
matrices. On the domestic side, it uses the factors calculated in the base year as
inputs, and alternative employment, land-use and cost data can be used to change
the volume and distribution of freight traffic. On the international side, alternative
costs, land use patterns and employment patterns can also be fed in to produce
alternative trip matrices but the total volume of trade does not change. Trade
forecasts are always exogenous in GBFM5, and fed into the program through the
main model, via a file generated by the FORK system, picked up through the
'Scenario' spreadsheet (cell F9). An equivalent, exogenous traffic growth mechanism
exists for the domestic sector in the main mode, relating changes in industrial output
to domestic tonnes lifted. This allows additional, unmodelled factors, such as
improvements in productivity, logistical changes or sectoral shifts to influence traffic
growth in addition to the three modelled factors; employment, industrial floor-space
and transport costs.
The 'RunForecastScenario' routine in 'MatrixBuild' only affects trips that use the
gravity model. The exception here is rail. Irrespective of transport costs, employment
and land-use, rail flows are added directly to the domestic traffic set without
modification.
3.5.3
EconLinks (STD)
EconLinks provides access to the information in the 'iotable_survey.csv' file, which
shows which industries are linked via the exchange of which products. The class is
constructed with a pointer to the file path. The constructor reads in the file, and an
access member is provided called 'GetValue'. The module will fail if SIC values
outside the range 0-100 are used.
3.5.4
Gravity (STD)
Gravity is also a simple wrapper class for a single input file 'gravityresults02.csv'.
The constructor causes the file to be read, and an access method is provided called
'GetRecord', returning a 'GravRec' structure, also defined inside the class.
Printed on 31/03/08
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