Cobweb of flows - The Visualization of Spatial Social Structure - page 169

Flows on and off the unemployment register are an aspatial set, which are
recorded. Here, within every employment area, only two states are given, moving
on or moving off the register. With a long time series, however, these little matri-
ces can tell us a lot about the frequency and average length of unemployment.
Flows of spatial movement abound and are usually categorised by purpose of
movement. Some have been mapped in the past, but we have little information
about many others: frequent flows to go shopping or to school for instance. Less
frequent, but important moves, to holidays, hospitals or further education, for
example, are also difficult to obtain data for or to estimate.
The only spatial flows regularly recorded and disseminated are those between
home and workplace, and migration from one home to another. Travel-to-work
flows are enumerated at the census (Figures 6.3 and 6.4). They link the basic
static points at which people are regularly enumerated; they give day and night
time populations and tell us how these change.
Migration is counted at a fine spatial scale by the census. A useful series can
be obtained from the National Health Service central register of patients. This
links everyone's NHS number to a specific General Practitioner and so records
when they move home as they tend to move doctor too. Migration flows have
been collected for many years to see how the night time population is changing in
the medium term. The creation of flow matrices was a by-product of this. 3 Here
all this is depicted as if a cobweb of migration was wrapped around a honeycomb
of social structure. The honeycomb is the cartogram. The cobweb is the flows.

6.3 Unravelling the tangles

If the movement from each state to every other state were indicated
separately, the multiplicity of lines would have made the map totally
illegible, but through combinations of migrations in the same direction
it has been possible to preserve legibility and still to show what was
intended.

(Thornthwaite and Slentz, 1934, p. 14)

Places in Britain can be divided into two: those where more people leave for
work in the morning than arrive to work (these are residential areas) and those
where more people arrive for work than leave (industrial (work) areas). Such a
map could have two colours, black and white. It would show us the rings and
sections of the basic commuting pattern. But how much of what we could know
would this very basic picture tell us?

3 Deviation from the expected propensity to migrate is mapped in some of the illustrations in this
topic: 'To discover any flows of unusual magnitude between certain areas, we calculated expected
migration flows. These prognoses are derived from the total out-flow from one area and the total
in-flow to another. (These form the expected cell frequencies that are also determined in chi-square
analysis.) The expected frequencies are subsequently compared with the observed frequencies (actual
flows). The difference reveals any unusual attraction or repulsion exerted by an area' (Jobse and
Musterd, 1989, pp. 247 - 248).