Maurice Kilbridge, Jon Didrichsen
ABSTRACTION, VERIFICATION AND VALIDATION IN
URBAN SIMULATION
The term "simulation", when used with reference to ab-
stract models, has come to have two meanings. One has
to do with the way in which the model is used as a sub-
stitute or imitation of reality, while the other has to do
with the structure of the model itself. When the latter
meaning is taken, as it is in this paper, it serves to
distinguish analytic models from simulation models on the
basis of mathematical structure and tractability. Analytic
models contain precise mathematical statements that can
be solved - at least theoretically - by standard mathe-
matical operations, while simulation models usually in-
clude statements about relationships between elements
which, though sufficiently clear and well structured for
conversion to a computer program, may have no mathe-
matical solution.
A simulation model is a formalized abstract system with
approximate rules that enables one to predict or describe
reality within certain limits. The power of simulation
rests in its ability to accept weak and inelegant theory -
a class into which most urban theory now falls. Descrip-
tive statements that can be reduced to the logic of a
computer program can constitute the model. A realistic
simulation model is one that comes as near to the system
of reality as we can get; that is, the theories built into
the model are valid assertions about the way in which
reality behaves.
Unlike the laboratory scientist, the urban planner can
seldom manipulate the objects of his study to find their
best arrangement or discover their natural properties or
laws. The scales of cost and time are usually too large
to allow experimentation with the physical elements of
planning, and controlled experimentation with the social
elements is rarely possible. By making a simulation model
to represent urban functions, the planner can create an
artificial envirönment for experimentation in which he
can test hypotheses or try out alternative public policies
and programs.
LEVELS OF ABSTRACTION IN SIMULATION
The development of a simulation model is an exercise in
abstraction, the removal from the real world of those
elements and relationships that are considered non-essen-
tial to the.theory of the model and the retention of the
essential. In urban simulation abstraction is achieved
mostly through aggregation and by assumptions that narrow
the range of situations. Aggregation is the grouping of
particular phenomena which share an essential common
characteristic into a single uniform set which is thereafter
considered and used as an entity. In the illustrative simu-
lation model presented later in this paper, the population
of an urban area is aggregated into five classes, employ-
ment into two, and land-use into six, thus reducing an
uncountable number of possible combinations to a manage-
able set. Abstraction by assumptions is achieved by
constraining the real world system in order to eliminate
uninteresting, irrelevant, or non-essential states of the
system.
With increasing abstraction the model becomes a more
and more structured and manipulable way of thinking
about real world phenomena, but it also becomes less
applicable, in that it can no longer pertain to all cases
at all times and under all circumstances. At some point
the gain in managability will no longer outweigh the loss
in applicability.
The process of abstraction in the development of a simu-
lation model forms a continuum with the real world at one
&nd and a computer program at the other. To provide a
systematic way of thinking about the process we will
divide this continuum into four sections. Each is a de-
finable and clearly conceived level of abstraction. Figure
] depicts these levels as a staircase starting from the real
world at the ground level, abstracting to a general con-
ceptual scheme, a manageable set of relationships, a
model and finally a computer program. We will now
examine each of these levels in detail.
From the Real World to a General Conceptual Scheme
The general conceptual scheme is the total theory about
how that sector of reality being modeled seems to behave,
It contains both systematic and intuitional knowledge
about relationships among elements of the city and tends
to be somewhat vague, qualitative and comprehensive. In
the illustrative simulation model to be described in the
next section, the general conceptual scheme is based on
the concept of gravity, the notion, deriving from an anal -
ogy with Newtonian physics that people and economic
ARCH+ 2 (1969) H.8