On the Problem of Choropleth Class Intervals

Abstract

The paper deals with possible methods of determination of class intervals in choropleth maps. The application of six chozen methods is used for the territory of Lower Egypt (the Nile Delta), which is divided into 61 counties (in Egypt named "markazes"). The selection of class intervals is based on frequency graphs. Two features are expressed by these graphs: the ratio of workers employed in agriculture and fishing (distribution A) and the ratio of workers employed in services (distribution B), both in percentage out of the total number of working population. The first distribution is an example of varying distribution, the latter one is more uniform and it does not differ too much from the Gaussian distribution. There is a close negative relationship between these distributions as it is shown by Fig. 3. Four class intervals are used for all six class interval scales and each interval is expressed in choropleth map by the same pattern. The limits of class intervals are selected by the following methods: 1. uniform arithmetic increase, 2. uniform division only in the section of great frequencies, 3. median and lower and upper quartile, 4. arithmetic mean and mean deviation from the mean, 5. arithmetic mean and standard deviation, 6. modal interval and other intervals of great frequencies. The number of counties in individual class intervals in shown by the tables; three limits of intervals are marked m1, m2, m3. One of the purposes of the article is to compare several methods and to show the advantages of various methods; namely the methods based on statistical calculations are important. In spite of this fact these methods are used in practical geography and cartography very rarely.