The natural breaks method of classification is based on the subjective recognition of gaps in the distribution, where there are significantly fewer observations. Plotting a histogram of the data can identify these gaps. This method, developed by George Jenks, minimizes variation within classes and maximizes variation between classes. This technique is most useful when the data set has more than one modal value. Once geographic features and data have been selected, generalized and classified for the map, it is necessary to choose the appropriate graphic representation or symbols for the information.
Symbols have characteristics that can be manipulated to suit the category of data being mapped.
These characteristics are referred to as visual variables or visual resources . These variables, individually or in combination, may be applied to map symbol design. However, not all variables apply equally well to the symbolization of all types of geographic phenomena or data sets. The symbolization of nominal or qualitative data is usually the least difficult. In this case, the symbol design should only indicate difference in class, and not imply ranking.