Isolines: These are constructed around urban centres to distinguish integrated from un-integrated areas. Isolines are lines around the centre in the same physical or time distance measured from the urban centre along the road system. Distance can be converted into time by referring to the average speeds for the various kinds of road links and transportation opportunities. Care has to be taken when calculating the points between roads where the transportation may take place on feeder roads or on foot paths.
In the case when isolines are used as an approximation for the service area, the isolines generation requires a decision on what might be called the maximum travel time for people outside the urban place to obtain a specific urban good or service. This crucial point cannot be left out. For orientation purposes, the average interaction values of the regions can be used. In every case it is useful to construct several isolines of different travelling distances. Areas within overlapping isolines of neighbouring centres are within the sphere of influence of two urban centres, with opportunity choices. Isolines cane can be drawn for the services such as secondary schools. In this case the maximum travel time for the pupils is used. In the case of a regional market centre, the travel time includes waiting time, change in the transportation mode and the real transportation time of farmers.
Accessibility models: Describe the present situation possible accessibility models as an entry step into the service area identification. Questions arising from the proposes model are then subjected to field investigation and analysis. By combining the model with the results of the field work specific conclusion regarding the accessibility between rural and urban areas can be ascertained.
According to the gravitation hypothesis, it is assumed that inhabitants living at any point between two settlements A and B will be attracted to the towns accordance with the relative attractiveness of the towns and inversely with the distance.
Distance can be measured in straight lines or better in road distances. In addition, cost and time factors can be incorporated. Attractiveness can be measured by the population size or the functional index.