Cobweb of flows - The Visualization of Spatial Social Structure - page 166

Box 6.1 Storing the flows

The full matrix of travel-to-work flows between the 1981 British wards
contained in theory 10 444 2 or over 100 million separate counts. It was not
possible to obtain flows within Scotland from the census records available.
Only 434 340 or half of one per cent of all possible routes were actually
travelled, by 20 602 790 people, as people work reasonably near to where
they live. The entire matrix of commuting flows was stored as a run length
encoded binary file of only 628 752 bytes in length. This was achieved by
sorting the flows from each ward in ward order and recording only the
displacement in ward number and size of the flow, each in four bits. A carry
option could be set if this size of field was inadequate. Thus the set of flows
from any ward could be determined instantly and the entire matrix easily
held in computer memory.
The migration streams were more dispersed and also subdivided by sex.
4 210 900 people moved across ward boundaries between 1980 and 1981,
resulting in 893 941 migration streams. It was stored in a file of 1 453 252
bytes (which would just fit on a floppy disk).

The final step in complexity taken here is to look at the change in flows.
Again, the information required at least doubles: the flows between all pairs of
cells at several points in time. The measure of how many more or less people
moved between two places from one pair of years to another provides an abstract
quantity, not easy to comprehend 2 (Figure 6.2).

6.2 What flows there are

In a time-space region, each individual can be visualized as a contin-
uous path starting in a point of birth and ending in a point of death.
Depending on the observation period, individual-paths can be referred
to as day-paths, year-paths or life-paths. This corresponds to the con-
cept of life-time in demography ... .

(Carlstein, 1982, p. 41)

Many flows are not spatial. The flow of votes between political parties pro-
duces a small matrix within each constituency or ward. Each element of the
matrix tells us how many people changed their vote from one party to another,
or chose not to vote. Often flows are not given; only net change can be derived.

2 It is the streams of migration that are real and of importance - not the marginal distributions
they produce: 'The simple framework that reduces migration to “pulls” towards desirable locations
and “pushes” away from less desirable ones cannot adequately explain even total migration, let alone
the spatial structure of streams' (Mueser, 1989, p. 196).