Geography Reference
In-Depth Information
44
Computer simulation and modelling of
urban structure and development
Paul Longley
INTRODUCTION
to be an 'urban system' is an important first step
in
urban modelling
.
Many of the other chapters in this topic have
used models as practical tools to understand
aspects of real-world systems. In the most general
terms, a 'model' can be defined as a 'simplification
of reality', nothing more, nothing less. In order to
answer the sorts of specific and focused questions
that were posed at the start of this chapter, it is
necessary to disregard information about aspects
of the urban system that are irrelevant (or only
partially or indirectly relevant) and focus on those
remaining system characteristics that have the
greatest impact upon outcomes. Thus a good
model
selectively
retains all of the aspects of a system
that are important from a particular point of view
and discards those that are not. This process of
selection is central to the art of model building.
In practice, the selected aspects of the system
are represented in an urban model principally
using quantitative data. Although such data are
used to represent system characteristics, they rarely
if ever do this in a way that is either perfectly
accurate or precise. This arises for a variety of
reasons which are common to all quantitative
analyses of socio economic systems. There is not
enough space here to explore these in much detail
(but see Martin 1997 for an introductory
overview of some important aspects). Briefly,
socioeconomic data (such as those from censuses)
are usually averaged across administrative zones
prior to being made available to researchers for
secondary analysis, and this creates problems of
Today's urban settlement structures are developing
and changing at a faster rate than ever before.
Applied geographers and planners are frequently
posed questions such as: What will be the effect
upon the existing retail structure of a town of the
development of a new out-of-centre grocery
store? What might be the effects of renewal
policies on house values in an urban area? How
will the density of population across a city change
if more central 'brown-field' sites are developed
in preference to peripheral ones? And what would
be the implications of either of these
developments for non-car-based transport policy?
Computer-based urban models and simulations
provide answers to these and other important,
explicitly geographical, questions.
These questions are fundamentally about
changes in the
form
of urban areas and also the
ways in which they
function
(Batty and Longley
1994). Applied and other geographers have long
sought to understand these problems by thinking
of towns and cities as
systems,
in which interrelated
geographical units or objects form a 'set' that
together function as a whole (Haggett
et al
.
1977:6). In reality, individual towns and cities (like
most all other geographical systems) do not
develop in isolation from the rest of reality, but it
is often helpful to screen out these more general
considerations in order to 'bound' the specific
problems that we are interested in. Limiting the
extent of what we consider for practical purposes
