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One final aspect of scientific geography that deserves mention is the way in which the various parts of the subject can be looked at as systems. Stoddart has described the system as a fundamental integrating concept in geography. A system can be defined as a set of objects related to each other through some form of circulation in which there are inputs of energy, circulations and finally some form of output. Such a system normally has a visible form which over a short period of time remains more or less constant, but over a longer period of time may well show variations which are adjustments to changing inputs. Any change in input may result in change of form. This will continue until a state of equilibrium, is again reached. Such an equilibrium is usually referred to as dynamic equilibrium, and indicates that there is no change of form despite a continuance of the working of the system. Many systems may have smaller ones working within the main system. Such smaller systems are referred to as sub-systems. The open systems have inputs and outputs of energy and mass. Isolated systems operate entirely within themselves without inputs or outputs of energy or mass, and are not met with in geography. In the case of closed system, energy exchanges take place, but no transfer of materials occur.

Haggett suggests that a world-wide study might be considered a study of a closed system, as important exchanges of energy do take place between the sun, the earth, and outer space system analysis has yielded insight into the structural characteristics and the functions and complex interacting phenomena. The systems concept provides an appropriate conceptual framework for holding substantive scientific problems in general and geographical problems in particular.

Models and theory. Many theories are, however, an integral part of a rather more structured representation of reality, normally referred to as a model. Haggett and Chorley have suggested that a model is 'a simplified structuring of reality which presents supposedly significant features or relationships in a generalized form' and that, as such they are valuable in obscuring incidental detail and in allowing fundamental aspects of reality to appear. Thus, models pick out the important generalizations in the working of any specific geographical system, and can be applied to all instances: A study of the principles behind any particular model does facilities an understanding of reality, and furthermore, allows prediction. In this way models are a vital part of planning.

Models can vary greatly in complexity; some may be so simplified that they carry no worthwhile information or at the other extreme, the model may be so complicated that it has no advantage over a description of all the complexities of reality-and no insight is conveyed.

Many models already exist in both human and physical geography. Some are introduced early in our geographical education. Models-in-physical geography are largely descriptive or are more concerned with process and evolution, as in the Davisian cycle of erosion. In human geography distance is an important part of most models, although it may not be necessary for a model to include the distance element for it to be considered geographical. The principle of distance decay is applied to gravitational fields, the mathematical relationship is often referred to as the gravity model.

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