Volume visualization - The Visualization of Spatial Social Structure - page 272

What do we do, though, if we wish to see how the four-way situation changed
over time? A slight change in the number of votes could send a division spinning
across the net, which in reality would hardly move in the tetrahedral space. This
would be unfortunate, unless only changes of party position were of interest.
What if there were a fifth party also of some importance, for example if the
green vote rose up in the future? Could we show a two-dimensional net of the
three-dimensional shadow of a four-dimensional hyper-tetrahedron? Or would it
be better to observe the rotating three-dimensional net of the four-dimensional
point cloud (Figures 9.17 and 9.18)? These situations are avoided for the while,
but remain to be addressed.

9.5 Flows through time

Rather than trying to simply display the data the idea is to extract
certain topological information and to display this ... a jillion little
arrows displayed in a cube would not reveal much about a three
dimensional flow.

(Nielson, Shriver and Rosenblum, 1990, p. 261)

If the possibilities at the end of the last section appeared a little daunting, con-
sider, for a moment, the problem of showing how a pattern of flows has changed
over a number of years - not a single change but a complete succession, ideally
going back some way in time. 11 Just such a truly three-dimensional matrix has
been constructed from the National Health Service central register, for flows (in
both directions) between every pair of ninety-seven mainland family practitioner
committee areas for each of the last fourteen years. 12 Even this low level of spatial
and temporal resolution produces over one hundred thousand separate counts of
migration streams. How can we begin to see what is happening to the flows of
people that create the spacetime pockets of existence (we call places) in Britain?

11 When you go back a little further in time you can easily be surprised. 'Particularly impressive
was the way in which the South East's population began to decline in the late 1960s, following its
rapid growth in the 1950s and the early 1960s' (Champion, 1989, p. 122).
12 Migration patterns have been fairly consistent over time, but do fluctuate; without images we
tend not to know much more than all these numbers: 'In 1989, the total number of moves between
FPC areas within England and Wales, at 1.76 million, was 6 per cent less than the 1.88 million in
1988 ... . There was little variation in the total number of moves during the years 1979 to 1985,
which ranged from 1.50 million (in 1981) to 1.60 million (in 1985). However, in 1986 the number
of moves increased to 1.83 million (a 14 per cent increase over 1985), with further increases to 1.87
million in 1987 and 1.88 million in 1988 . During this period, expansion of financial services,

resulting in easier access to mortgages, and relatively low interest rates may have contributed to the
increased number of moves. Similarly, the fall in the number of moves in 1989 could have been
partly due to the rise in interest rates. The total number of moves in 1989 was still 13 per cent above
the average for the seven years before 1986' (Bulusu, 1990, p. 33).

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