Cobweb of flows - The Visualization of Spatial Social Structure - page 172

Elaboration can begin by moving from two colours to a continuous colour
scale. The image is smoother. However, just how many people are moving? The
proportion of people moving out of each place can be shown, or that of those
moving in, or both. This can be done with the colour of the cells, or their size,
using size for indicating the proportion involved and darkness for the proportion
outgoing, for instance.
Next, consider where, in general, people are going to and coming from. This
is implied by the urban, suburban and rural rings of similarly shaded areas in the
map being described here, but the patterns may not be as simple as we imagine. 4
Arrows can be drawn showing the average direction of outgoing movement by
changing the shape of each cell to be an arrow, one that points to where the
people are travelling (Figures 6.5 and 6.6).
After direction we can consider how far people are moving or, better still,
how long it is taking them to get there. The length of the arrow is another aspect
we can alter. The length is in proportion to the average time it takes people to
travel; the direction indicates where they go, the size shows how many of them
go and the colour - say through its level of saturation - depicts what proportion
is outgoing.
The succession of images just described has been concerned with average
flow. Such maps are useful, for instance in studying commuting, because most
areas are either residential or industrial and most people go in roughly the same
direction and travel the same amount of time. It is only the nature of travel to
work that allows it to be mapped like this (Figures 6.7 and 6.8). The arrows
could not be used to show where people were coming from or how far they
travel to get to the shops, for instance, as these aspects are too variable, being
concentrated in the centre of cities and of large neighbourhoods. However, it is
better to show them as straight lines rather than flows along winding roads. 5
The images shown here highlight the most important flows, but many flows
are not shown at all. 6 The patterns seen are partly an artefact of the precise

4 ' ... in some cases, the number of in- and outcommuters are almost the same so that they
should be represented by the same size of circle. If the same size of circle is used for each, there is
no rim left to indicate which group predominates. If it is attempted to show a rim, then the graded
circles are not accurate. The main advantage of this method, however, is that in- and outcommuters
can be represented on a single map' (Dale, 1971, pp. 17 - 21).
5 'Tracing the actual itineraries is not sufficient for representing a system of relations. A map of
maritime routes, even when weighted, does not show the direction of trade among the centres of
activity; it shows the density of ships at sea. The maritime trade among the cities of Europe and
the Mediterranean will only appear in its diversity, weight, and geographic direction, when each
connection, even through maritime, is represented by a straight line ... ' (Bertin, 1983, p. 344).
6 'The cartographic difficulties involved prevent the representation on one map of all net move-
ments from each state to every other state, yet, since it seemed necessary for purposes of comparison
to get as much of the movement as possible on one map, it appeared desirable to affect a certain
compromise. It was found that the optimal threshold for the deletion of entries is the average

flow size. This data selection rule deletes as much as 80 % of the flow arrows, but generally only
20 % of the flow volumes' (Tobler, 1987, p. 155).

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