The urban sphere of influence can be defined as the geographical region which surrounds an urban system and maintains inflow-outflow relationship with the system .
Every urban centre, irrespective of the size and the nature of function, has a region of influence. Generally speaking, as the size of the population increases, the multiplicity of functions increases. As a result, the influence zone is larger and vice versa.
The term sphere of influence area was first used by Northam and supported by Canter. Other terms to express a similar entity, which have got recognised, include umland and city region. Umland is a German word which means the area around. The term was first used by the Allies in the Second World War.
The term city-region was first used by Dickinson. It is used to describe a similar situation on a much larger scale. Some other terms which have become popular include urban field, tributary area and catchment area. The term sphere of influence is preferred by political geographers.
Delineating the Sphere of Influence Area:
Several methods have been worked out by geographers and sociologists, but no single method seems to be perfect.
The pre-First World War geographers depended primarily on empirical methods (through questionnaires and field surveys) taking into account all those relevant functions which are performed by cities and the surroundings of the city. The influence zone of each function is first delineated. It brings out the multiplicity of boundaries of spheres of influence area.
Harris has suggested that a common boundary is to be drawn from within those boundaries which are very close to each other. Harris himself drew a sphere of influence area for the Salt Lake City of Utah State in USA. He used 12 important services for this purpose which included retail trade, wholesale grocery and drug sale, radio broadcasting, newspaper circulation, telephone services, banking distribution etc.
Harris scheme shows greater dependence upon the services of the cities. He practically ignored the services rendered by rural areas. Geographers like Carter, Dickinson and Green studied the sphere of influence area and their empirical methods gave due weightage to the rural services.
The post-Second World War geographers began to use statistical methods. This made the inferences more precise, logical and scientific. This method, however, has the disadvantage of being rigid. Still, it is a popular method throughout the world.
The conclusion of the method brings the delineated influence area closer to Christaller’s observations, who suggested that every urbane settlement (service centre) is supposed to have a hexagonal influence region. It solves the problem of existence of shadow zone which normally appears in the case of spherical delineation of the influence region.
The statistical method is based on the principle of gravitation. Reilly propounded the Law of Retail Gravitation to delineate the market zone of urban centres. Since marketing is a principal function, this method is used by geographers to delineate the zone of influence area.
This method states that:
P= MA x MB / d2
where MA = Mass of centre A measured by population size, such that MA > MB
MB = Mass of centre B
d = distance between two cities.
The result will mark the distance of the sphere of influence area from Mass (city) A; the remaining distance will mark the influence area of Mass (city) B. Modern urban geographers give importance to this method as they consider this cut-off as an important factor for development of respective influence areas.
Some development authorities have begun to use the sphere of influence area as the basis of regional planning. They use detailed questionnaires to understand the nature of influence. They consider factors such as daily commuting, functional structure of village, household types of villages, milk supply, vegetable supply, newspaper circulation etc. This approach seems to have some practical utility.
It gives due weightage to natural hindrances. Factors like rivers, mountains, forests, marshy lands etc. are bound to modify the influence area and in that case, the statistical method is not of much relevance. Information collected through questionnaires is, however, properly processed through different statistical methods and a composite index, indicating a common boundary, is worked out. This common boundary gives the limit of the sphere of influence area.
Thus, the sphere of influence area is highly relevant in socio-economic patterns of a city and its surroundings. In India, the regional planners have given due recognition to the role of city regions or spheres of influence areas in the ‘Growth Pole’ strategy adopted by the Planning Commission of India in the Sixth Five- Year-Plan.