An unclassed choropleth is designed to show statistical data in a multi-colored value moving from smallest to largest, using lighter colors to portray smaller values and using progressivly darker shades to portray progressivly greater values. Examples of an unclassed choropleth might include: how many head of cattle ar being sold per far per county within the state of Nebraska. Another example might be what the average rainfall in inches was for the farmland of Colorado per county.
Choroleths are maps that generalize specific statistical information, into averages that can be understood graphically. Although choropleths are generalized they do provide an excellent overview of information distribution.
Choropleths display information in three primary elements. They are: 1) size and shape. 2) number of classes shown. 3) class limits methodologically.
A good color pattern should progress from lighter to darker portraying smaller statistical units to larger, this may include monochromatic or color applications as both show a progression of color differentiation. The size and shape of a choropleth should form a pattern that is discernable, the number of classes is delimited by the amount of data being represented.
Click here to a bad example of a choropleth
This is a bad example of what a choropleth should ideally be as it has an unnatural color progression and does not show any trends in the desired information when there clearly should be.
Click here to see an example of a better choropleth This is a better example of a choropleth as it does provide a better sense of pattern associated with the use of color to display information. However it does not quite meet all of the requirements that are necessary to the production of a quality choropleth. The problem with this choropleth is that it doen not move from lighter to darker with its color progressinon and this causes some dificulty in the interpretation of the information.
This is an example of a grey scale shading progression. It begins with 20% black and progresses to 100% black as its various shadings. This is a fine pattern to use when creating a choropleth when either color is not available or not desirable to use.
This is an example of a color progression that is an acceptable method when using color to show an incremental change in the statistical data. The yellow is the lightest and lowest valued in data and the progressively darker shades show higher statistical values.
As you can see the previous two examples are gradual, subtle changes in color and are easy to follow from least to greatest. This is the optimal configuration for a choropleth and should be the goal of the cartographer as he/she crafts their choropleth.
Choropleths can be an extremely useful tool when trying to understand more about statistical data within a geographic area.
Brought to you by the folks in Dr. Michael Peterson's Cartography class. Goodbye and happy choroplething!!!!
