1 Spatial Modelling of the Terrestrial Environment - Georeferencial

Modelling the Impact of Traffic Emissions on the Urban Environment 229
Figure 11.1
The integration of project modelling components using the SHIRE 2000 GIS
Given these assumptions a framework for modelling emissions impact has been created
and is shown in Figure 11.1. The framework combines three distinct models dealing with
land use, transport and emissions.
The GIS acts as a powerful integrating tool for maintaining and managing all of the
model input and output data. It also provides a basis for analysis and visualization of results.
Table 11.1 shows that there are three broad categories of primary information maintained
within SHIRE 2000 in the form of image products, cartography/census geography and
socio-economic data. There is also a substantial volume of ancillary datasets and intermediate,
analytical results that would be almost impossible to organize effectively outside a
reliable database management system.
The land use and transport models provide an effective basis for predicting the movement
of vehicles by mode on the County transportation network. They function as a closely
coupled module that is capable of representing the feedback relationships between changes
in land use and changes in the transportation network through time. They have been developed
over a period of some 25 years (see Echenique, 1994) and have a proven track
record in large-scale modelling of complex, urban systems. In addition to modelling of the
actual situation or ‘reference case’ that is a product of past planning policy, they also enable
prediction of the outcomes for a range of ‘policy cases’ which are the results of alternative,
proposed policy packages.
The emissions model was developed within the framework of the ArcView GIS using
the methodology set out in DMRB (1999). For the purposes of this study it has the benefits
of being widely recognized as a robust screening model with relatively modest data input
requirements. It takes predictions of vehicle numbers by mode and average speed for each
transport network link from the land use/transport modules and it converts these into maps
of emissions concentration on a 25-m raster grid covering the county-wide study area.
Having provided a broad overview of the infrastructure created for emissions impact
modelling, a more detailed description of the land use, transport and emissions models will
now be provided.