Area Maps

Understanding and Using Choropleths, Isopleths, and Area Maps


Choropleths and isopleths function as common forms of area maps, which are geospatial visualizations. But they have two key differences that separate them: choropleths rely on boundaries, while isopleths rely on density.

A choropleth map, or filled map, takes a set of data that applies to a country, a state, a county, a parish, or any sort of geographical location and uses shades of color to denote the relevant values. They are great for presenting data on elections, population density, median income, or any other measure that can be tied to specific locations with boundaries. The first choropleth map was created by Baron Pierre Charles Dupin in 1826, a French engineer and economist who tried to map the levels of illiteracy across the regions of France.

Isopleth maps don't rely on a defined area like choropleth maps do. They overlay the data on a map following contours and areas of interest independent of arbitrary boundaries and borders. Radar maps used in meteorology are one of the most widely viewed examples of an isopleth area map.

How do I read it?

A choropleth map will display different values or percentages of a data set in the geographical location. A map that shows the number of births per county in a state defines the boundaries of the data. Counties with a higher percentage of births could be assigned a dark color, while counties with lower birthrates could be identified by a lighter color.

Isopleth maps rely strictly on continuous data. The isolines, or boundaries, that appear on a map connect data points using the same measure. This means that isolines of equal value appear on an isopleth map with values on one side being higher than the values on the other side. The same color is used to fill an area when your ranges possess a similar value. Isopleth maps are similar to choropleth maps in that they color the areas between isoline.This type of map allows the reader to see the gradual changes occurring on a map due to the absence of strict boundaries.

What type of analysis does this support?

Choropleth maps are useful in that they can present the actual numbers behind your data on a map, however due to their use of colors they don’t need all numbers to be present on a map to identify geographical hot spots. Most viewers will wonder how the data set applies to where they live, or they will want to look for trouble spots in geographical locations. These maps measure percentages as parts of a whole best. A choropleth map is a simple way to visualize a measure across a geographic area by displaying the range of variability through the use of color.

Isopleth maps work when you do not want your data bound to predefined boundaries like a choropleth maps. If you want to use continuous data to show gradual change over a geographic region, an isopleth helps the viewer see where the center of the change is located. Weather maps are kinds of isopleth maps because they aren’t bound by states or population numbers. However, patchy data that doesn’t connect to other points does not work well in an isopleth map.

When and How to Use Isopleths and Choropleths for Visual Analysis

If you have a regional or population based pattern from one set of data, a choropleth map can show the differences in different areas easily to a viewer. Isopleths, on the other hand, don’t need a set boundary to show anything on a map.

You should use a choropleth map if:

  • Your data can be shown through percentages of a whole on a map
  • Your data can be divided into ranges
  • Your data shows just one variable
  • Users don’t need to see specific numbers or variables

You should use an isopleth map if:

  • Your data isn’t bound by location or population
  • Your data doesn’t need to be shown in proportions to a whole

When designing these maps, here are some things to avoid:

  • Don’t make the key focus of the map a region that is too small to be seen
  • Don’t use conflicting colors or more than one color scheme (yellow and blue is confusing)
  • Don’t try to show more than one data set on a map

Some good alternatives to these maps include cartograms, symbol maps, and dot density maps.

An Example of a Choropleth Used Well


This choropleth map follows the obesity rates per county throughout the United States. But rather than assigning a new shade of orange to every different rate, the rates are split up into ranges.

  • One consistent color theme is used throughout the map
  • The ranges give a better idea of which regions suffer the most from obesity
  • There is only one category measured over all

Ineffective examples of a Choropleth and Alternative


POOR EXAMPLE

This choropleth map shows the same categories as before, but with one key difference that makes it a bad map. Each number for each county has a different color. The overwhelming number of colors/counties makes it impossible to determine patterns or regional rates.

BETTER ALTERNATIVE

A better alternative for this map is an isopleth map, where the data isn’t split apart by boundaries. The location of the regions with the highest problematic areas is the center of the splash of red, while orange radiates around the red.