Some Questions
 What would induce emigrants to leave everything behind
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and risk everything on moving elsewhere?
What considerations influence how individual human
beings use space and act within it?
Are there discernible controls on human spatial behavior?
How does Distance affect human interaction?
How do our perceptions of places influence our spatial
activities?
How do we overcome the consequences of distance in the
exchange of commodities and information?
How are movement and migration decisions reached?
 Spatial Interaction: the movement of peoples, ideas,
and commodities (goods that are bought and sold)
within and between areas.
 Some familiar examples: International trade,
semitrailers on the expressway, radio broadcasts, and
telephone calls.
 Movement of whatever nature represents the attempt
to smooth out the spatially differing availability of
required resources, commodities, information, or
opportunities.
Bases of Interaction
 Neither the world’s resources nor the products of
people’s efforts are uniformly distributed.
 Commodity flows are responses to these differences.
 Matters of awareness of supplies or markets, the
presence or absence of transportation connections,
costs of movement, ability to pay for things – all and
more are factors in the structure of trade.
 Edward Ullman (1912-1976): observed that spatial
interaction is effectively controlled by three flowdetermining factors:
 The three factors:
 Complementarity
 Transferability
 Intervening Opportunities
Complementarity
 For two places to interact, one place must have what
the other place wants and can secure.
 Restated: one place must have a supply of an item for
which there is demand in the other, purchasing power
with which to acquire it, and the means to transport it.
 The word describing this circumstance is
complementarity.
 Just having different stuff though… not enough to
initiate exchange.
 Brazil and Greenland have completely different
resources, but have almost no interaction because of
lack of desire for goods.
 The movement of crude and refined petroleum
between spatially separated areas clearly demonstrates
complementarity.
 More generalized patterns of complementarity
underlie the exchanges of raw materials and
agricultural goods for money and industrial
commodities.
Transferability
 Even when complementarity exists, spatial interaction
occurs only when conditions of transferability are met.
 Transferability: acceptable costs of an exchange.
 Spatial movement responds not just to availability and
demand, but to considerations of time and cost.
 Transferability is an expression of the mobility of a
commodity and is a function of three interrelated
conditions:
 1. the characteristics and value of a product
 2. the distance, measured in time and money, over which it
must be moved
 3. the ability of the commodity to bear the costs of movement
 If the time and money costs of traversing a distance are
too great, the exchange does not occur.
 Mobility is not just a physical matter, but an economic
one as well.
 If the cost is too great on arrival to a buyer, trade does
not occur.
 The buyer would either go without, or find a
substitute.
 Transferability is not a constant….
 It differs between places, over time, and in relation to
what is being transferred, and the manner in which
the movement occurs.
 The opening of a logging road will connect a sawmill
with previously inaccessible timber.
 Increasing scarcity of ore will enhance the
transferability of lower-quality mine outputs.
 Transferability expresses the changing relationships
between the costs of transportation and the value of
the product being shipped.
Intervening Opportunity
 Complementarity can be effective only in the absence
of more attractive alternative sources of supply or
demand closer at hand or cheaper.
 New York won’t buy sand from the Sahara desert
region because it has a closer, cheaper supply more
locally.
 For reasons of cost and convenience, a purchaser is
unlikely to buy identical commodities at a distance
when a suitable nearby supply is available.
Measuring Interaction
 The study of unique exchanges/events, is suggestive,
but not particularly informative.
 We seek general principles that govern the frequency
and intensity of interaction both to validate the three
preconditions of spatial exchange, and establish the
probability that any given potential interaction will
actually occur.
 We are looking for aggregate, not individual, behavior.
Distance Decay
 The lives and activities of people everywhere are
influenced by the friction of distance.
 Because there are increasing penalties in time and cost
associated with longer distance, the exchange is more
expensive, and therefore less likely to occur.
 You visit nearby friends more often than distant
relatives.
 Students in college order out food when they are near
the restaurant, but don’t if they live further away.
 Most interactions occur over short distances.
 Interchange decreases as distance increases, a
reflection of the increase in transferability costs.
 Distance Decay: describes the decline of an activity or
function with increasing distance form the point of
origin.
 It is also evident however, that the amount or rate of
distance decay varies with the type of activity.
 Distance isn’t as important as time and cost though
when determining the rate of distance decay.
 When the friction of distance is reduced by lowered
costs or increased ease of flow, the slope of the
distance decay curve is flattened and more total area is
effectively united.
The Gravity Concept
 Interaction decisions are not based on distance or cost
considerations alone.
 Larger, regional shopping centers (like a mall) attract
more people because of the variety of shops and goods
that its size promises.
 You go to big cities to seek your fortune, as opposed to
the nearest small town.
 We are attracted by the expectation of opportunity
that we associate with larger places.
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 Henry Carey: (1793-1879) wrote Principles of Social
Science. He observed that the physical laws of gravity
and motion developed by Sir Isaac Newton were
applicable to the aggregate actions of humans.
 Newton’s law tells us that big things attract each other
more than do small objects, and things that are close
have a greater attraction then things at a distance, and
that the attraction decreases greatly with even the
smallest increase in separation.
 Carey was interested in the interaction between urban
centers and in the observation that a large city is more
likely to attract an individual than is a small hamlet.
 He took Newton’s formula, and exchanged physical
mass for population size.
 Exchanges (E)AB = Population of A * Population of B
Distance between A and B
 In social (such as Carey’s work), rather than physical
(Newton), distance may be calculated by travel time or
travel cost modifications rather than by straight line
separation
 We know two things from this:
 1. That exchanges decrease as one increase distance from
the source (in this case, an Urban area)
 2. That in a stationary place, exchanges will increase as
the size of the source increases.
 William J. Reilly: (1899-1979) Proposed the law of
retail gravitation in 1931
 Reilly determined the relative amount of retail trade
that two cities would attract from an intermediate
place in the vicinity of the breaking point.
 What Reilly is saying is that any farm or small-town
resident that is located between the two bigger cities,
would be inclined to shop in one or the other
according to that resident’s position relative to the
breaking point.
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City A (pop 70k)
BP
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Farmer M
City B (pop 150k)
 The gravity model can be used to account for a wide
variety of flow patterns in human geography.
 Population migration
 Commodity flows
 Journeys to work or to shop
 Telephone call volumes
 Etc…
Interaction Potential
 Distance decay models and gravitational pull models
only deal with two places…
 In reality, the world is rather more complex.
 All cities, rather than just two, in a region have the
possibility of interacting with each other.
 In addition to that, the more specialized each city
becomes, the greater their collective complementarity,
and the more likely that multiple interactions will
occur
 A Potential model, which is also based in physics,
estimates the interaction opportunities available to a
center in such a multi-centered network.
 It gives the relative position of each point in relation to
all other places within a region
 This model is applicable whenever intensity of
interaction is of concern;
 Marketing
 Land values
 Broadcasting
 Commuting patterns, etc.
Movement Biases
 Distance decay and the gravity and potential models
help us understand the bases for interaction in a
perfect world… one with no natural or cultural barriers
to movement.
 Once flow patterns develop, they tend to cement
themselves in…
 i.e. A shopping center attracts people, merchants see
increased flow as desirable and open new shops, so even
more people go there to shop, etc…
 Such an aggregate regularity of flow as displayed by
the previous example is called a movement bias.
 We know of distance bias, meaning people prefer short
trips to long ones, but there is also direction bias…
 Of all possible directions of movement, actual flows are
restricted to only one or a few.
 Direction bias also implies that from a given origin,
movement is not random:
 In the U.S., most tractor trailer flow is along an East-
West axis, even though from Kansas you could go any
direction.
 Such directional biases, like the east-west flow of semi
movement in the United States, is a partial reflection
of network bias.
 Network bias is a shorthand way of saying that the
presence or absence of connecting channels strongly
affects spatial interaction.
 A set of routes and the set of places they connect are
called a network.
 This is not limited to moving goods, but also
information
 Everything we have talked about, especially these
biases, move us away from the three bases of spatial
interaction and aggregate behavior, and towards more
individualized movements and behavior.
 The questions we ask in terms of spatial interaction,
lead us to how much refinement we need.
Human Spatial Behavior
 Mobility: term applied to all types of human territorial
movement.
 There are two types that concern us:
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Daily or temporary use of space – going to the store, work,
school, etc, as well as longer periods such as vacations or going
to college.
Longer term commitment such as permanently leaving the
home and finding residence in a new location.
 The second type is known as migration.
 Groups and countries draw boundaries around
themselves, divide space into territories, and defend
them if necessary.
 Territoriality: the emotional attachment to and the
defense of home ground.
 On a more individualized basis, we each claim
personal space, the zone of privacy and separation
from others our culture and physical circumstances
require or permit.
 Besides having our home territory, we also have a
home range, known as “activity space”, an area in
which we move freely on our rounds of regular activity.
 The following map shows the probably activities for a
family of 5 for one day.
 Note: activities for the group cover a small area, and
individual, the area is even smaller.
 Note: over a longer period of time, more paths would
have to be added, extending the area covered.
 The types of trips and thus the area of their activity
space, depend on at least the following three variables:
 Stage in life course (age)
 The means of mobility at their command
 The demands or opportunities implicit in their daily
activities.
The Tyranny of Time
 All of our activities consume time as well as space.
 Your spatial reach is restricted because you cannot be
in two places at once.
 There is also a finite amount of time within a day, and
your spatial choices have to take this into account.
 Our daily space-time constraints may be represented
in a space-time prism.
 Critical Distance: the distance beyond which cost,
effort, and means, strongly influence our willingness
to travel.
 Since most activities have their own time constraints,
the choices of things you can do and the places you can
do them are strictly limited.
 Defined class hours, travel time from home to school,
among other things, may be constraints on your spacetime path.
 What jobs you can take are limited by what can fit
within your daily space time prism…
 To the right is a “Space-
time path” for a
hypothetical college
student.
Answer the following on a sheet of
paper…
 What is meant by “spatial interaction”?
 What is activity space? What factors affect the areal
extent of an individual’s activity space?
 Plot your “space-time path” on a typical class day.
What alterations in your established movement habits
might be necessary if
 (a) you rode a bike instead of walked?
 (b) instead of riding a bike you drove a car?
 (c) you had to take a bus instead of going by bike, foot,
or car?