Some Questions What would induce emigrants to leave everything behind 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 ( ): observed that spatial interaction is effectively controlled by three flow- determining 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.

` Henry Carey: ( ) 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: ( ) 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. | City A (pop 70k) BP 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: 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 space- time 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?