Transportation Geography and Network Science/Spatial Interaction
Spatial interaction is a dynamic flow process from one location to another.(Figure 1)  It involves a wide range of flows between nodes: these include human movements (e.g., migration and commuting to work) and movement of materials (e.g., international trade).  For example, there is always an exchange of material, energy, people and information between cities to ensure the normal operation of production and life. At the regional level, interactions include many modes and types of flow, such as rail, air, highways, and even pipelines. In cities, spatial interaction takes a wide variety of forms and includes commuting to work, travel for shopping and other travel purposes.
Human activities and their interactions in time and space are the driving force of many social and economic phenomena. Human’s normal lives, including work, social, shopping, entertainment, etc., require a certain amount of resources. These resources are often only available in certain areas. Therefore, people must exchange time for space to carry out certain activities. In order to reduce the time spent, humans have created various modes of transportation systems and established the form of the city, concentrating the interactions in a relatively small space.
On the one hand, spatial interaction can strengthen the links between relevant regions. Exchange or expand the space for development, and hence obtaining more development opportunities. On the other hand, spatial interactions may cause competition between resources, elements, development opportunities, etc., and may cause damage to some areas.
The concept of spatial interaction was first proposed by the French geographer Edward Ullman in the early 20th century. According to Uliman’s conception, there were three basic conditions for spatial intersection: complementarity, transferability and intervening opportunity.  According to the form of spatial interaction, P. Haggett proposed a classification in 1972. He used three methods of heat transfer in physics to classify the form of spatial interaction into convection, conduction and radiation. 
Three Bases for Spatial Interaction
Initially, it was believed that functional differences between regions were the conditions for an interaction. However, Ullman believes that from the perspective of supply and demand, the interaction between the two locations requires a precondition that demand or deficit at one location and supply or surplus at another. Ullman called this relationship Complementarity.
For example, in China, the large rural population in second- and third-tier cities provides the labour source for the first tie cities, where the labour population is insufficient. Alternatively, sawmill requires logs, while a forest provides them. Complementarity is like a potential gradient of goods and people flowing from a high energy state (in the excess state) to a lower energy state (in a state where the state is in a deficit state).
The complementarity between the two places has led to the movement and circulation of goods, population and information. However, it may also be the case that when the goods are transported between A and B, the two places intervene in C site that can provide or consume goods. As a result, the intervening opportunities are created. It causes the replacement of the origin of goods. At this time, even if there is complementarity between A and B, the interaction is difficult to produce.
In general, Intervening opportunity serves two purposes:
- It can save on transportation cost. Assuming the transportation cost is directly related to the distance between the origin and destination. If location B and C provide the same good to A. If the distance between C and A is much shorter than the distance between B and A, C will act as an intervening opportunity as it is cheaper to transport goods from C to A. As a result, the goods from location C will be eventually preferred.
- It has a filter effect that affects transportation, especially on population movements, which leads to displacement at locations and reduces long-distance spatial interactions.
Intervening opportunity is usually considered as the reason for the lack of interaction between the two complementary sites. Complementary will only generate a flow if there is no intervening, or closer, location. Otherwise, the flow of goods that occur between two complementary sides maybe diverted to a third location, if it represents an intervening opportunity. Therefore, a cheaper complementary alternative lead to a cheaper transportation cost.
Transferability refers to the cost of overcoming distance measured in real economic terms of either time or travel cost. It is affected by the following factors:
- Spatial distance and transportation time. The longer the spatial distance and the transportation time between regions, the more inconvenient to make economic relations. Consequently, the input price will increase, so the transferability will be poor. The cost of overcoming distance is known as the ‘friction of distance’. If the spatial distance between the two locations is too long and the cost of overcoming the distance is more than acceptable. The interaction will still not occur in spite of a complementary supply-demand relationship.
- The transportability of the goods. Transportability is closely related to the economic distance of the object being transported. In general, the transportability of goods is determined by the value of the unit weight. The transportation distance of low-value goods tends to be short and the transportation distance of high-value goods is longer. For example, heavy, low-value goods such as concrete blocks are usually being transported over a short distance, whereas the high value and low weight goods such as diamond are transported globally.
- The geo-culture and geopolitical issues. If there are economic protection barriers, cultural isolation, political and social conflicts or conflicts between regions, then transferability is poor. For example, China and the United States are engaged in a trade war. Both sides are adding tariffs on the import goods from each other. The existence of a trade war led to a significant decline in the transferability of both countries.
- The convenience of transportation. If the transportation method is more diverse and faster, then the transferability is high.
In addition to the impact on the transportation of goods, transportability also has a significant impact on people's shopping trips. People usually take fewer roads to buy low-value goods and take more roads to get high-value goods, which leads to the emergence of a commercial centre hierarchy.
Forms of Spatial Interaction
The forms of spatial interaction are divided into three types: convection, conduction, and radiation.
- Convection: It is characterized by the movement of materials and people. For example, the transportation of raw materials manufactured goods between the place of production and the place of consumption. And the transportation of mail, parcels and the movement of people.
- Conduction: It refers to various transactions between cities. Such as financial transactions between cities, etc.
- Radiation: It refers to the flow of information and the diffusion of new ideas and new technologies.
As a result, spatial interactions are manifested in three main forms: the movement of goods and people, trading processes, and the flow of information.
The interaction of space requires the use of various transportation and communication tools. The movement of matter and population pass through various transportation networks, such as the railway network, the road network and the aviation network. The transmission and exchange of information must pass through different communication networks, such as the mobile phone, radio, televisions and internet. Therefore, it is also possible to classify interactions based on various networks on which the interactions take place. If the network and the city are considered simultaneously, then the city is a clustering point for networks. The more networks interspersed in the city, the better the transferability of the city, the stronger the role of the city.
Limitations on Ullman's Theory
Ullman proposed three conditions for spatial interaction in 1956, and he discussed mostly the flow of physical materials. In comparison, there were fewer discussions on currency flows and information flows. The impact of the evolution of industrial organizations on spatial interactions was not mentioned. For example, the flow of money is less affected by the law of friction of distance. Today, with the highly developed means of communication and the formation of global financial networks, international financial services can operate 24 hours a day. With the development of economy and society, the position of money flow and information flow in spatial interaction will become increasingly important, so it is necessary to further study their unique characteristics.
The form of spatial interaction maintains the normal production and life through exchanging and connecting materials, energy, people, funds and information between cities and regions. It varies widely in direction, distance and time. Regardless of different kinds of interaction, it consists of three basic elements: the emigration location, the immigration location and the flow route between the two locations. These three elements are the basis of spatial interaction. Millions of interactions have this basic form, and numerous spatial interaction units overlap each other and are intricately intertwined. It makes the cities or towns of different grades and functions of different levels have a close relationship and form a certain structure. Spatial interaction is the most active and richest part of human space activities. It is always in the constant movement, constantly recombining, constantly forming a new spatial dynamic world, providing extremely important for the development of geography. The level of spatial interaction reflects the level of economic development to a certain extent. The level of economic development also reflects the degree of division of the regional economy and whether the economic structure is reasonable.
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