Choropleth


 * Choropleth mapping**

Chropleth maps are one of the most widely used thematic map forms today. This type of mapping involves the symbolization of enumeration units, such as census tracts, counties, or states, using area patterns or graduated colors.

For example, a cartographer may need to map, by ZIP code area, the percentage of households that reside in rented apartments in Huntington Beach, California. The cartographer determines the number of households renting apartments and the total number of households within each ZIP code area. The percentage of households living in rented apartments, for each zip code area, is calculated with the following formula:

( Total households renting apartments / Total number of households ) * 100

The results are grouped into five classes. The class with the lowest percentage of apartment renters is displayed with a light blue, the class with the highest percentage is displayed with a dark blue, and the three classes in between are displayed with intermediate shades of blue, changing gradually from lighter to darker as the percentage of renters increases.

Major concerns are the method of data classification, areal symbolization, and legend design.


 * General rules for choropleth mapping are as follows:**
 * Use data that are assumed to be uniform throughout an enumeration unit.
 * Because enumeration units vary in size, //do not map totals//. Use derived values, such as ratios, rates, proportions, or percentages. Population densities (total number of persons/area of enumeration unit), for example, are commonly mapped using choropleth techniques.
 * The best classification method depends on the data, the map reader, and the purpose of the map..
 * When classifying data, the full range of data must be included and class values should not overlap. No more than six classes are recommended.
 * Class symbols (i.e. colors or patterns) must be easily distinguishable.

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