What is a Choropleth Map?

Choropleth maps use differences in shading to express values across a geographically defined area. The coloring is based on the numerical values, and can be expressed in intervals, quartiles, or with normalized variables. Choropleth maps have been around since 1800s while being one of most useful visuals for plotting data in geographic space.

Below, is a simple choropleth map showing the unemployment rate across the United States. This examples shows changes using a lights blue to dark blue color scale. Color scale show the distinction of areas in choropleth charts.

For other visual check out my posts on Bubble Maps, Radial Charts , The Best Alternatives to Pie Charts.

Keys Ways to use a Choropleth Map

They have been around for over 200 years, and in that time they have found plenty of useful applications. Some of the more common uses are showing unemployment, average salary, home prices, and population details. One of the great aspects, as long as we have geography data we can throw it into the map.

Although there are preferred suggestion to follow to use the chart like a pro. Firstly, using’s the lowest level of data will drastically improve your chart. For one, showing as much detail as possible will highlight true hot spots and trends across the map.

Additionally, detail allows for flexibility. Lower level examples like zip code, town, or city can always be rolled up to a higher level. Its up to the creators discretion on what tells the most accurate story. Unfortunately though, lower level information in a lot of cases isn’t available.

The second suggestion revolves around the choice of metric to use. Using absolute values, raw number or totals will almost always show us the distribution of the population, and highlight more densely populated areas. Showing populated areas isn’t a meaningful or insightful data story! To avoid this mistake, we can normalize are measures using percentages, ratios or rates.

Advantages of Choropleth Maps

If you are debating on using choropleth maps, there are several advantages to using this type of map and visual.

1. Choropleth Maps are Easy to Create, and are Standard in most Reporting Applications

For example, this map is a standard option in Excel, PowerBi, and Tableau. Within these programs there are already several built in customization to handle variation of this map. We can add labels, use data groupings, change out color schemes without pretty easily. The unemployment example I used was created in less than a minute.

2. Easy to use to understand geographical trends for analysis

While doing data analysis its hard to see emerging trends driven around areas. This is even harder if your not familiar with the area your analyzing which is very likely to happen. Plotting data on a choropleth map or almost any map can quickly give you a sense of areas that maybe driving the data. Location in some instance is arbitrary, but there could be factors driving those results.

While spotting for trends the maps will highlight hot spots in the data leading to more questions. These questions are great for data story telling and

3. The map can be used at various levels geographically

There is no limitation on the area that we need to plot. We can have the map showing counties, all the way up to countries. This partly caused by the extensive applications choropleth maps are in. Plus, some application have different map layouts that could be used.

4. Easy for the audience to understand

well this could be a benefit depending on if the chart is well thought out. The audience your preparing the chart for will most likely have some understanding of the geographic area. Choropleth maps when well labeled are intuitively easy to understand. If there are important factors to showcase regarding the location of trends this visual can be very effectives. Although this could go under a disadvantages for certain examples.

Disadvantages of Choropleth Maps

Whenever we make a decision to use a visual, we need to understand what are potential risk and downsides. A map is aesthetically pleasing, but it could be the wrong visual to use for a certain project. These are some of the disadvantage to consider before using a choropleth map.

1. Geographic boundaries could have nothing to do with your data

Like most charts, the choropleth map doesn’t tell the full story. The number will be summarized based on geographical borders. It is possible that we could see large changes between one area and another, and those changes could just be random coincidence.

Plus, if there is change from one area to the next the coloring represents it like its the whole area. That’s usually not the case because

On the other hand, the overall graphic doesn’t explain why a geography is different than the other.

2. Losing Details Depending on the Level of Aggregation

For this chart type we always have to pick a level of aggregation. Even on some of the lower levels we will lose detail. For example if we build a county unemployment map we could have some highly unemployed towns that wont be reflected in the visual. The projects needs to be okay with losing granularity.

3. Limitations on Metrics that Can Used

Metrics need to be transformed, and normalized in choropleth maps. There will instance where totals are the preferred metric to use.

4. Color scales are easily skewed by outliers

The coloring a choropleth map will be driven by higher values. If there are value that sku high, the rest of the areas will be color a similar shade. This is an issue because there could be meaningful differences between these values that will not be easily seen in the charts.

Conclusion

Choropleth maps are useful visual tools. There are things to consider when using them. For the most part, its helpful to make draft to see if it will be useful.

For other visual check out my posts on Bubble Maps, Radial Charts , The Best Alternatives to Pie Charts