What is a Choropleth Map? Choropleth Map Advantages and Disadvantages
What is a Choropleth Map?
A choropleth map is a type of map where areas are filled in with color to show density or another statistical measurement. It is a map that uses shading or color to indicate the statistical value of a geographic area. They are commonly used to display information such as election results or demographic data.
A choropleth map is created by shading or coloring a geographic area in proportion to the value of a statistic that is specific to that area. Choropleth mapping is a great way to show data distribution over a geographic area.
The data for this type of map is usually aggregated within geographical units like census tracts, counties, states, or countries. Maps can be either shaded or colored with different tones to represent the density of the population in that area.
One example would be the most populated cities in the United States by color intensity (darkest shades go to most populated).
History od Choropleth Maps
Baron Pierre Charles Dupin developed the first known choropleth map in 1826, demonstrating the availability of elementary education in France by district.
In France, other “cartes teintées” (“colored maps”) were quickly made to display additional “moral data” on education, sickness, crime, and living circumstances.
Choropleth maps soon acquired popularity in a number of nations as the availability of demographic data derived from national Censuses increased, beginning with a series of choropleth maps published in the official reports of the 1841 Census of Ireland.
After 1850, when chromolithography became readily available, color was increasingly applied to choropleth maps.
The term “choropleth map” was coined by geographer John Kirtland Wright in 1938 and was widely used by cartographers by the 1940s. Glenn Trewartha reintroduced them as “ratio maps” in 1938, but this title did not persist.
Choropleth maps are incredibly popular, and they are most likely the most frequent type of themed map in use today. That’s a positive thing since it suggests your audience will probably comprehend them.
Because most of our geodata is provided by enumeration units, such as census data, we are accustomed to thinking of the world as split into geographical units such as census tracts, counties, and provinces.
Most cartographers, however, would argue that choropleth maps are overused and frequently misapplied if the geographic phenomena being mapped aren’t integrally linked to enumeration units.
For example, contagious illnesses, soil types, or age demographics are unconcerned with county lines or zip codes, and they rarely alter abruptly at such man-made boundaries.
Tax rates, on the other hand, are extremely tightly related to enumeration units, vary suddenly, and make excellent sense as a choropleth map. A choropleth map makes less sense the less the object you’re mapping is related to enumeration units.
Choropleth Maps Classification
Since these types of maps are used on such a wide range of data, it’s helpful to have an idea of what choropleth maps are capable of.
Since each choropleth map is unique, the limitations depend on the variables you are using.
The following rules apply most often to the colors used in choropleth maps:
- There should be at least three colors for your map.
- If there are more than three colors, the map should visually represent this by using larger areas of color, rather than by shading.
- If there are more than five classes, you should use shaded symbols instead of colors to differentiate the classes.
- When you have a large number of classes and no longer need level lines indicating class boundaries, you can dispense with shapes altogether and instead draw straight lines between each class.
Classification Methods are:
The range of values is divided into equal intervals or an arithmetic progression such that each class has an equal range of values: (max – min)/n.
For example, the above-mentioned salary range ($20,000 – $150,000) is separated into four classes: $52,500, $85,000, and $117,500.
The dataset is divided into quantiles such that each class has an equal number of districts. For example, if the United States’ 3,141 counties were classified into four quantile classes (i.e., quartiles), the first class would comprise the 785 poorest counties, followed by the next 785.
When the number of districts does not split evenly, or when identical values cross the threshold, adjustments may be required.
Natural Breaks is a type of “optimal” classification technique that discovers class breaks that reduce within-class variation and maximize between-class differences for a given number of classes.
One disadvantage of this technique is that each dataset creates a unique classification solution, and if you need to compare across maps, such as in an atlas or series (e.g., one map each for 1980, 1990, and 2000), you may want to use a single scheme that can be applied to all of the maps.
We frequently need to manually arrange one or all of the class breaks. For instance, are there certain break moments that must be “hardwired” into your class breaks?
Is it necessary for one of the classes breaks to be the mean one? Is this map part of a series that requires the same classes to be used across all maps (such that the colors always relate to the same numbers on every map)?
Do any of the other ways bring you near to a good solution that might be enhanced with a few minor changes to those classes? If this is the case, do not be afraid to schedule these class breaks yourself.
Choropleth Maps Color Progression
If you have an opportunity to choose your color scheme, pay particular attention to the way in which the colors move from one class to the next.
As we discussed previously, variety is a key aspect of choropleth maps.
But there are a few rules that must be followed when developing these maps:
- The classes should not share boundary lines or fill patterns, or both.
- The colors should be continuous, so that no highlights stand out from the surrounding color.
- The colors for one class should not necessarily match those for the next class in a progression of colors.
- There should be a progression of colors from one class to the next, as if moving down a spectrum, rather than jumping all over the place within each class’s boundaries.
The most popular color progressions used in choropleth (and other themed) maps are as follows.
- Sequential progression- Variable values are represented as color values in a sequential progression.
- A qualitative progression- employs a strewn collection of colors in no particular order, with no intended distinction in value. In a qualitative choropleth map, this is most typically used with nominal categories such as “most prominent religion.”
- A single-hue progression- is one that fades from a dark shade of the selected color (or gray) to a very light or white shade of the same hue. This is a typical way for mapping magnitude. The darkest shade denotes the most numbers in the data collection, while the lightest shade represents the fewest.
- A partial-spectral progression- utilizes a restricted range of colors to provide more contrast to the value contrast, allowing for the employment of a greater number of classes. Because of its natural perceived lightness, yellow is usually selected at the lighter end of the sequence. Yellow-green-blue and yellow-orange-red are common color ranges.
- A spectral progression- It employs a broad variety of colors (perhaps the full color wheel) with no intentional value distinctions. When there is an order to the values, but there is not a “more vs. less” order, such as seasonality, this is most usually utilized.
Non-cartographers commonly utilize it in instances when other color progressions would be far more effective.
Choropleth Map Types
There are three types of choropleth maps:
Single-value – A single variable is used to classify the data. Example: unemployment by county. This map is often used to show the frequently changing value of a single variable over time.
Dual-value – Two variables are used together to divide the map into classes, often using a common theme or concept. Example: unemployment rate and median household income by county.
Area-based – Area or region-based data is used to mark geographic areas on the map, often using the land area of a single county to define a portion of the map and using different colors for those areas. Example: population by city.
Choropleth Map Examples
Let’s look at a few examples of choropleth maps in action.
Let’s consider an example from the movie: “The Big Lebowski”.
The Big Lebowski: A map of the United States in which the states are labeled with colors as in Lebowski’s apartment, many of which are based on significance. This uses a sequential progression of colors to represent values.
The Lebowski Map: A map of the United States in which the states are labeled with colors as in Lebowski’s apartment, many of which are based on significance. This uses the same sequential progression of colors to represent values as above.
The Dude Map: A map of the United States in which the states are labeled with colors as in Lebowski’s apartment, many of which are based on significance. This uses a sequential progression of colors to represent values.
Choropleth Map Uses
The uses are:
- To identify data unique to a study, or that is not found in other data sets
- To answer a question about a geographic area
- To compare two areas or decide on the relative importance of factors
- To determine how districts are distributed across different land areas, as in Congressional district maps
- To communicate the general idea of data distribution as an impression that may not be as precise as you would like it to be
- To communicate the idea of data distribution as an impression that may not be as precise as you would like it to be.
Choropleth Map Vs Isopleth Maps
A choropleth map shades or colors an area (usually a county or state) to indicate a measure of something, such as population density. The regions are usually colored or shaded to indicate a quantity.
For example, a map of the United States might shade each state a different color to indicate the unemployment rate.
An isopleth map, also called a line map, shows points of equal value on a map, typically using lines to connect points of equal value.
The lines are usually drawn thicker or in a different color to indicate the quantity. For example, a map of the United States might connect points of equal population size with a thick line.
Choropleth Map Advantages and Disadvantages
Choropleth Maps Advantages
Choropleth map advantages include:
- Conveys an idea of geographic distribution with minimal effort
- Truly qualitative data can be classified to show differences in quantity and marked as such in the legend
- Most easily understood and read by many people
- Thematic maps are more visually pleasing and attract the eye, which is good for marketing
- Maybe scaled or modified easily
- Maps are hard to fabricate
Choropleth Maps Disadvantages
Some disadvantages are:
- A single color scale may not provide enough contrast between values, and can lead to values being misinterpreted as one another if the difference between them is not great enough (i.e., purple and red appear similar even though they may depict totally different values)
- May be difficult to use with other data sets that are not as easily classified
- A single orange dot in a choropleth map may be interpreted as a single county if it’s too large. This can result in a coloring error such as a white area being labeled as part of the colored area.
- May confuse people into believing there is one correct answer, when in fact there are many valid answers
- May confound correlation with causation, which is to say that the relationship between variables may not be clear cut
- May be misinterpreted if too much emphasis is placed on a minimum and a maximum level, rather than highlighting the area around each value as suggested by a choropleth map. This may have disastrous results such as marking all flying objects orange, or all snow white.
- May be inaccurate if incorrect or unverified data is used, which can cost time and money to fix
- A choropleth map will probably show the most obvious patterns in data distribution. This will lead to the appearance that there are other patterns that do not exist, or that a pattern exists where it does not (this is known as Simpson’s Paradox, or the “doughnut effect”)
- A choropleth map relies on human perception to depict differences in values accurately.
Choropleth Maps Application
- It is applied in Cartography to indicate the percentage /quantity of specified units with respect to the number of units in said category (e.g., distribution of rainfall over a certain period of time).
- It is also used in Geography for showing information about the different demographic divisions like – age, religion, sex, and nationality etc.
- The Choropleth map can be used to display any graphical representation or variation of data that is classified according to a defined scale mentioned in its key domain (i.e., Age group, Area, distance etc.
- In Mathematics, it is used to depict the total number of words in a sentence.
- It can also be used to depict the distribution of the different values within a set to its respective classes (i.e., each class should denote the total of all values within that class). The map is usually represented using different colors such as Red, Blue, Purple or any other color depending upon the number or class of said variable being depicted.
- In Literary criticism, it illustrates an imaginary space requisite for a story to occur. The space is usually represented as a concentric circle around a single location. However, under some cases, the space may be depicted on the map with a different color depending on the location of that specific point.
- In Statistics, it is used to depict the percentage of individuals within an area or population who possess a certain trait or characteristic. It also helps in depicting any trend that is shown within the data set (i.e., percentages and values).
Choropleth Map FAQs
What is a choropleth map?
A choropleth map is a type of map that uses different colors to show the statistic for each region. The values are often notated so it is easy to compare regions and see which has the highest statistic.
Choropleth maps are often seen in geography textbooks to represent population, but can also be used to display other data such as time zones or countries.
What is a choropleth map used for?
A choropleth map is a type of map that depicts color on a scale to show the categories of data. It is used as an easy way to represent quantitative data.
It visualizes information that is usually hard to understand or visualize such as the number of people within different age groups or if there are any major religious faiths present in a certain area.
When is a choropleth map used?
Choropleth maps are used to show data distribution, which helps in identifying certain patterns in the data. These maps are also used to depict trends in data and help the viewer arrive at correct conclusions about the general area of interest.
How do I create a choropleth map?
Choropleth maps can be made manually or with statistical software. They usually include colors that represent different values for each category of data. The user can make the map manually by drawing an outlined shape for each category of data and coloring them according to the values that pertain to each category.
How do I read a choropleth map?
Choropleth maps are usually easy to interpret because they display specific categories of data. The colors represent different classes of data, which means that if the color is red, then that class contains all values within that class (i.e., red signifies all numbers with the value of 6).
If only one value is present, then the color will be white (i.e., all aces in a 3-card poker game).
Why should I use an AIA Choropleth Map over others?
There are many versions of choropleth maps, but for government agencies, it has been shown that the American Institute for Aeronautics Choropleth Map is the most versatile. It can be easily customized and scaled to show any type of data.
What kind of data do Choropleth maps show?
Choropleth maps can show any distribution of data. It should be noted that it is more likely to show domains with a relatively large number of values and a small number of categories (i.e., large data sets).
It is also used to represent areas with a large percentage of a certain group as opposed to all individuals within the area, as in age groups.
When should I not use a Choropleth map?
There are instances when you should avoid using choropleth maps. They are especially not ideal for use with discrete data.
This type of data is different from the numerical data that choropleths represent. They are also not ideal when mapping data such as health information, because they do not usually have a clear and single color to represent a specific value.
What is an example of a Choropleth map?
The CDC uses choropleth maps to show the distribution of obesity across the United States. Each state is represented by a different color.
Is a Choropleth map better than a dot map?
Choropleth maps do not show all of the information that a dot map does. For example, they do not show the exact details of locations, nor do they represent points on a grid.
It is more popular than the dot map because it can show the distribution and divisions of data more clearly and easily than dot maps can.
What is Simpson’s Paradox?
Simpson’s paradox is also known as the “doughnut effect”. It occurs when the data shows a trend that is contrary to what one can deduce.
What are breakpoints on a choropleth map?
Breakpoints are a different type of point on a choropleth map than those that are normally shown. Breakpoints occur when the data is brought to a certain point in relation to the plotted values.
What is a radial choropleth map?
A radial choropleth map shows how many people or items within an area are categorized using color categories of nine or fewer. This is a very ideal type of map when used in conjunction with the other types of choropleth maps.
It allows for more organized and clearer comparisons between the varying categories of data.
What are the advantages of using a choropleth map?
The main advantage is that they can be easily displayed using different colors. This makes it easier to make comparisons with others.
A choropleth map is a map that illustrates data via different shading or hatching according to the statistic being measured. The name comes from the Greek words for “map” and “color.”
A more specific type of choropleth map is called a density-equalizing choropleth, where data are summarized by geographic area, with each area shaded according to the value of the statistic at that location.
Isopleth maps, on the other hand, are made up of lines that connect points of equal value making it easy to see data points that are spread out across a map.
An isopleth map is a map that uses lines to connect points of equal value on a map.