Three-dimensional modelling is an attempt to represent the world in three dimensions, simplifying through deliberate assumptions. In archaeology, this has developed as an extension of the traditional use of three-dimensional drawings to help present and record data. The debate in the archaeological literature over whether surface or solid modellers should be used is one based on the premise that the purpose of three-dimensional modelling is data visualization. This concentration on perception modelling has been at the expense of research on the modelling of structure.

Modelling is an attempt to represent the real world. This model
can be either a static one, which represents a part of the world
as it is in one particular instance, or a dynamic one, which attempts
to model the way that things *happen*, rather than *are.
*In the process, it is necessary to make simplifications, which,
if they are performed adeptly, require that explicit assumptions
be made about the real world. These simplifications enable us
to work with something which is simpler than the real world, and,
it is hoped, will remove a lot of the noise of insignificant detail.
This will enable us to see the elements which we consider to be
significant beneath. We then have a model with which we can work
and, hopefully, by our assumptions we have also made ourselves
more aware of how we think the model relates to reality.

Modelling is not the same thing as taking a photograph, because the simplification from real world to photograph is an accidental product of the photographic process, while modelling is a deliberate, explicit transformation of the real world to a copy of it. It is not possible to construct a model which does not involve assumptions about the way in which the world is put together. 'The existence of a model presupposes the existence of an underlying theory, since a model is but one simplified, formalized and skeletal expression of a theory' (Clarke 1972b, 3). A model is thus not an expression of our perception of the world, but of our attempt to understand that perception.

A three-dimensional model is simply any model whose approximation
of the real world incorporates the third dimension. It is thus
a very broad category, and in a spatial discipline such as archaeology,
almost all modelling should eventually fall into this category.
Although in principle a computer three-dimensional model need
have no graphical output (Atkin *et al.* 1987, 1), in practice this is today the case, because it is impossible to conceptualise all but the most basic spatial data from numbers alone.

Archaeology, as a discipline concerned with material culture,
has a great need for visual information simply to describe its
data (as opposed to interpreting data), as with the development
by the hard sciences of Data
Visualization techniques. It has a long history of the use of hand-drawn three-dimensional drawings to facilitate the description of a site, or enable the
reader to visualize a proposed reconstruction. Although three-dimensional
data such as stratigraphy or a building can be represented as
a series of plans and elevations, this representation of the data
is one stage further divorced from the real experience than would
be the case if the data were represented in three dimensions.
We are able to conceptualise our data better and work with it
more efficiently and accurately if we can see it in three-dimensions.
'A design that is represented by a fragmented arrangement of drawings,
specifications and site investigation reports has a habit of creating
additional work' (Atkin *et al.* 1987, 59).

Buildings archaeology is a particular case where three-dimensional representation of the data, both for publication of results and also for recording and research, is of great benefit. As the quality of recording in buildings archaeology improves, and the questioning of the data becomes more demanding in terms of accuracy and detail, the traditional two-dimensional methods are proving to be inadequate for the wealth of data produced and required. In his proposal submitted to the National Endowment for the Humanities in October 1994, John Dobbins (1996) wrote that, 'Traditional methods are insufficient for efficiently and accurately recording, storing, and relating the evidence that is quantitatively large, architecturally complex, and three-dimensional in nature. The existing two-dimensional 1:100 plan of the forum [at Pompeii]... cannot support the abundant three-dimensional data that prompt the new research questions'.

It is very difficult, and often very dangerous, to manipulate three-dimensional data unless one can visualize it in some way. This is because our brains can conceptualize visual data much more readily and proficiently than mathematical data. Therefore, almost all three-dimensional models will have graphical output. Because the initial needs for computer-based three-dimensional modelling in archaeology were in the fields of representation and recording, the graphical nature of these models became entrenched in the archaeological mind-set. I believe that it is as a result of this that research in the archaeology of the built environment using three-dimensional modelling has focused on perception modelling (e.g. Reilly 1992). The appearance of the thing has become more important than its underlying structure.

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Last updated: Thu May 1 1997