Geographically separated populations can be treated as compartments. In fact that can frequently be done in many ways so an important question is what are the best ways of defining the compartments. An apparently simple way is to divide a map area with a grid and treat the population within each grid box as a subpopulation. That is simple but is usually extremely naive; the only circumstance in which it is truly applicable is when the geographic and population substrates are both relatively homogeneous. One could also take administrative divisions such as counties or areas defined by geographic boundaries as the basis for a compartmentation. That may be useful in some cases because poulation data are often available on that basis but in general, it is not an astute way to go about that. The important thing to realize is that the best compartmentation is often problem dependent. Thus for spread of diseases, one wants to define compartments in relation to the transportation routes that form the basis for population movement between different areas. Let us illustrate that with a simple example.
Let Figure 5 represent a major town that is a shopping center for smaller villages (4 in our example) spread throughout a surrounding farming area. The large town has a northern suburb where the well-to-do live.
Figure 5
Here R10 is a primary road; in addition there are secondary roads throughout the region, many leading from the small villages to R10. A useful compartmentation for spread of an infectious disease such as a respiratory tract infection might be to make the major part of the town one compartment and the suburb another. However, if the town is large and further divided socio-economically, one might divide it into more than one compartment. In addition make each of the villages with its surrounding dependent farm families a compartment. Then the flows from one to the other could be defined in terms of the traffic between them, taking into account the difference between daily traffic and the traffic on market days and weekends.
With the introduction of a disease, each population subgroup would then be divided into compartments for the stages of the disease process, e.g. susceptibles, infecteds, immunes.