Spatial interactions include different types of movements such as travel, migration, transmission of information, journeys to work or shopping, retailing activities, or freight distribution. Our knowledge about spatial interaction can be generally or highly particularistic (specific). Normally, it seems easy to state why a particular movement takes place: the heart of a household goes to work, students go to classes or lectures and the house wife goes to the super market. People movement seems to be obvious for some purpose that it seems a bit silly to ask- why do people move? However, different interaction model can provide answer or useful explanations; they can allow predictions as well as description of past events. Explanations of region interaction pattern permits their social control and suitable approaches that can advance information on general welfare.

What is spatial interaction?

Spatial interaction is the flow of products for example, goods, people, services and information among places in response to localize, supply and demand relationship that is often expressed over a geographical space. It is the movement of people, goods, and information between the points of origin to the point of destination. Spatial interaction usually include a variety of movement such as travel, migration, transmission of information, journeys to work or shopping, retailing activities or freight.

The basis for spatial interaction is created or improved only after there is a demand which the new system will satisfy. For instance, it was not until many settlers had come to United State that the regular scheduled steamship service was established in America and Europe. In Nigeria, long distance bus services (mass transit) have been established to link up the 36 states including FCT.

Transportation improvement foster increase interactions but may be widely responsible for them. The French geographer Edward Louis Ullman; perhaps he is the leading transportation geographer of the 20^th century, more formally addressed interaction as complementarity, transferability and lack of intervening opportunities According to Ullman, three conditions affects transportation which are also known as the-

Three principles of spatial interaction

1. Complementarity-(a deficit of a good in one place and a surplus in another),
2. Transferability-(possibility of transport of goods or products at a cost that the market will bear)
3. Intervening opportunities-(where similar goods or products is not available at a closer distance.

• Complementarity– traditionally, it was assumed that interaction between places developed because of area differentiation (the fact that places differ from another). Although, this is true to a certain extent, but mere differentiation never produce interaction by itself. For example, we have only to consider the many different part of the world which have no interaction of any kind with one another. For two places to interact there must be a demand in one place and a supply in another. The demand and supply must be specifically complementary. For instance, In Nigeria, cola nut growth in the south is transported to the north probably it is largely consumed in the north, while in the north Groundnuts and livestock are being transported to the south. However, crude oil and refined oil/petroleum products and commodities of relatively low value per unit of bulk move enormous distances from the fields (in Niger Delta in Nigeria) to the Canadian prairies and Venezuela to industrialized areas/countries of Europe, Japan, and North America. Without specific complementarity of supply and demand these regions, the movement and interaction will not take place.
• Intervening opportunities– it is important to note that complementarity between places can generate exchange only in the absence of intervening opportunity. For instance, if we are considering the patterns for movement of goods from point A to B, we have to also consider another place known as point C between them which might acts as an intervening origin or an alternative destination. For example, Lagos is closer to Ibadan than it is to Jos. For Lagosians to travel for Christmas break, Ibadan constitute an intervening opportunity between Lagos and Jos. Intervening opportunities doesnt always curtail long distance interaction; it is entirely possible that such a sequence of opportunities can help to create interactions between widely separated transport links profitable and thereby paying part of the cost between distance places. This is what happened in the construction of trans-continental railway road in USA.
• Transferability– this is concerned with the fiction of distance measured in real time and money cost. If the time and money cost of traversing a distance are too high, the movement will not take place despite perfect complementarity and the absence of intervening opportunities. Instead of reaping the benefit of interaction or maintain their status quo and refuse to exchange the life they know. Thus if the goods cannot be moved because of high cost of movement, other goods will be substituted if possible or with just so without. Transferability differs over space between classes and modes of movement.

Examples of spatial interaction

The main examples of spatial interactions include:

1. Movement of people
2. Movement of goods and services
3. Movement of information

Some places are well endowed with skilled people, physical resources, main spring of knowledge and new ideas. Elsewhere, supplies are scares or demands are great. Severe inequalities from place to place can results in interaction if the three principles of spatial interactions are satisfied- lack of intervening opportunity, complementarity and transferability.

Why do people move?

World population is concentrated in few places and shows little tendency to spread into empty areas.

1. Movement of people– in every age and at every spatial scale, population are on the move in a continuing effort to reach the peak of every changing opportunity surface. It is the difference between present and perceived alternative opportunity that triggers decisions to move; when the perceived difference becomes great and no obstacles stands in the way, then migration takes place. For instance African especially Nigerians migrating to Europe on foot through Sahara desert to Italy, France, Germany and also the influx of women into Italy for prostitution and change of status and the tendency to earn more money.
2. Movement of goods and services– this involves the movement of goods and services from point of production to the point of consumption. In the movement of goods, a product moves from the factory to the consumers so as to maximise its net delivery price. For his reason, the factory serves as the nearby market first to save transport cost, then serving those farther away. All commodity and service movement can be evaluated in terms of complementarity, supply and demand areas, presence or absence of intervening opportunity and transferability of goods and services.
3. Movement of information– information consists of new facts, ideas, data and routine information for example, people, goods and services. Information flows from place to place, moving from place of production to demand areas. Those who sent the most messages get most of the return contacts. A tendency exists for flows to run from places of abundant to areas of effective demand, and effective pulls are exacted at each destination.