Cobweb of flows - The Visualization of Spatial Social Structure - page 182

all the wards of Britain are being shown, as there is too little space to use the
visual ordering technique effectively or to draw anything other than all thin lines
(Figure 6.11).
Width can be used to convey magnitude of flow and arrowheads to convey
the direction. The fewer the number of areas between which flows are being
drawn, the more effective these methods can be. Colour can be reserved to show
other things about those who are travelling: what kind of areas they are coming
from or going to and what kind of people they are. The length of the line is a
good surrogate for the time taken when the population cartogram is used as the
base. It tends to take longer to pass by more people. This base also makes the
picture clearer and gives some meaning to the density of lines (Figure 6.12).
So far, by looking at a relatively constrained set of flows we have not encoun-
tered all the problems this kind of mapping can create. It should be noted that
the picture produced is very sensitive to the denominator used (Figures 6.13
and 6.14). Next, by adding colour, more can be shown. Then, by considering
also migration, another web of long term movement can be mapped upon the
first diurnal flows.

6.5 Commuting chaos

Programming the mapping of other than the most crude versions of
such flow maps is not trivial, not least because large flows tend to
occur between areas close together on the ground and numerous lines
occur if all flows are mapped. Most frequently, they are mapped by
arrows whose width is proportional to the flow involved.
(Rhind, 1983, p. 176)

What the commuting flows show us is the well-known city structure of Britain
and the extent to which this is a true interpretation. On the population cartogram
the flows appear much more confused than it is suggested they are when plot-
ted in supposedly normal (Euclidean) space. This brings the image closer to
the messiness of reality, 11 but in reality only a minority of travel flows are of

commuters.

the characteristics of his new neighbours. Correlations and slopes between 1966 census data and
partisanship change so little when using election years other than 1966 that we would be surprised
to discover large biases due to the ageing of the census' (Miller, Raab and Britto, 1974, p. 399).
11 Visually, ordering lines according to spatial proximity relieves some of the confusion: 'There is
a particular problem in using line segments to connect locations on a map: the geographically longer
lines are visually dominant. In many data sets, such as migrations or trade flows, the flow between
locations varies inversely with distance. One consequence is that, at times, short but important lines
may be difficult to see because of the long, less important lines. One possible way to alleviate this
problem is to use wide lines for connecting nearby locations' (Becker et al ., 1990a, p. 289).