Cobweb of flows - The Visualization of Spatial Social Structure - page 180

Box 6.2 A significant flow

A flow between two places is of a significant magnitude if it is larger than
you would expect, given the populations of the two places and the general
propensity to move between places. Here a flow was deemed to be significant,
and thus drawn when

n n

m ij s t m jist
p it p jt

j m ij s t i

>

n

(n − 1 ) p it

i

where m ij s t is the flow from place i to j between times s and t ,and P it is
the population who could move at place I at time t ( n is the total number
of places being considered). The equation therefore calculated whether the
geometric mean propensity to move between two places is greater than the
expected propensity to move.
Problems can occur when this method is applied to commuting flows,
particularly in central London, where many people are moving into an area
of generally low night-time population. It can be useful to use the day-time
population estimate (caused by the flow) as the denominator.

of flows counted. This can have as great an effect as does the number of people
who are actually moving. 7
More people will flow from larger areas, but less people will move between
such areas (since more move within them; see Box 6.2) and more will flow
from long narrowly shaped areas than from more compact places. Put simply,
the longer and wigglier the boundaries are over which flows are recorded, the
more will be recorded. 8
One solution to the problems raised here is the same as earlier: to use as
many small areas as possible and to record as many of the flows that occurred as
possible. Here wards represent the finest resolution. With flows, however, long
distance movements can come to dominate the image, stretched across many
other areas and perhaps visually concealing more important flows. Two solutions
are used - one visual, the other statistical.

7 'Migration is an event which by definition involves two places - even if only adjoining houses.
So spatial location and spatial units are more basic to migration than to other events such as births,
deaths and consumers' expenditure. For these latter events, space is not fundamental to the event
itself: it is merely necessary to define the geographical limits of the population to be included ... '

(Craig, 1981).
8 '

for areas of the same size out-migration rates would be higher for a long and narrow areal
unit than for a circular one' (Duncan, Cuzzort and Duncan, 1961, p. 34).

...