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migration does have a strongly localised tendency. 12 If it did not do so, as we
see later, it would be practically impossible to map by the techniques used here
(Figure 6.17).
The more serious problem of flow mapping is that there need not be a single
strong net direction of flow. Flow can and does occur in both directions. How can
bidirectional movement along a line be depicted? Various methods can be used
when the flows are only between a few dozen places and the arrows are still large
enough to have specific characteristics. Placing the smaller flow as an arrow on
top of, and in the opposite direction to, the larger flow is the solution preferred
here. Net flow is then represented by the differences between the arrows, as it
should be, being a difference and not an entity in itself. 13
To show net flows only would be simpler, but it would also be highly mis-
leading. The majority of the movements would be largely cancelled out. Interest
should primarily be in how the change occurs, in how many people move and
where - not in a difference. 14 In fact the net flows in migration are very small
in comparison with the absolute movements. Migration flows between two areas
tend to be roughly equal in both directions. This fact can be used to advantage
when drawing images of migration between many areas. 15
One solution that has been suggested 16 is to re-route flows by the shortest
path through contiguous areas. This was tried, but it was found that for Britain
the effect was to imply the opposite of the true picture. Much of the flow went
through the middle of the country and the relationship between the metropolitan
cities and their hinterlands was reversed. The idea was abandoned.
12 Here we are reaching the limits of what can be sensibly drawn: 'There is no doubt that graphic
complexity would be enormous if large populations in sizeable regions for a long time period were
to be drawn as paths; the picture of merely one day in a small village is quite complicated even if
computer plotters were programmed to do the actual drawing. But the important task of the graphic
notation system is not to thrive on visual complexity, but to reveal the under-lying logic of human
society and ecology in space and time' (Carlstein, 1982, p. 45).
13 Some migration counts can include the same person several times, so: 'Although the number
of moves into and out of the country during the last ten years was close to 5 million - equivalent to
nearly 10 per cent of the total population - this does not mean that 5 million different people were
migrating' (Davis and Walker, 1975, p. 2).
14 ' ... there seems to be a tendency for self-employed and managerial people to move rather less
than many of their subordinates, especially over short distances. This might be termed a social effect,
reflecting position in the hierarchy of status and power; at longer distances, more economic effects
become stronger, so that the poor move much less often than the comparatively rich' (Hollingsworth,
1970, p. 62).
15 The illustrations drawn here show clearly how the top third of the class structure dominates
long distance migration: 'The majority of labour migrants are middle class, in the 25 - 44 age group,
and have middle-high incomes. They are professional and managerial workers in a career structure
that encourages movement. Their occupational status frequently entitles them to financial and other
forms of aid which make migration easier. They move within a housing market that is especially
geared to their requirements' (Johnson, Salt and Wood, 1974, p. 246).
16 Tobler (1987).
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