Thematic Map | Definition, Types & Examples
Table of Contents
- What Is a Thematic Map?
- Types of Thematic Maps
- Thematic Map Example: A Thematic Map of the USA
- Lesson Summary
What are thematic maps with examples?
Common thematic maps are choropleth maps and graduated symbol maps. They are often used because of the ease of use for the map reader, which enables the cartographer better convey their message. A choropleth map example is the coloring of counties across a state or country based on their population size. A graduated symbol map would map the same data using a symbol sized proportionately to the data amount and placed within each county on the map.
Why it is called a thematic map?
It is called a thematic map because the map has a specific theme. The theme corresponds to the data selected, and the data is connected to a particular geographic location. This data and geography connection makes this type of data geospatial data.
What are the most commonly used thematic maps?
There are a few commonly used thematic maps. The most frequently used thematic maps are choropleth, dot distribution, graduated symbol, isoline, cartograms, heat, and flow-line maps.
Table of Contents
- What Is a Thematic Map?
- Types of Thematic Maps
- Thematic Map Example: A Thematic Map of the USA
- Lesson Summary
Thematic maps are a common type of map. A thematic map is used to illustrate the relationship between a theme and physical space, such as trade patterns, the spread of disease, or whether someone says pop or soda. It is essential to know how to read thematic maps because they offer condensed package information.
Thematic Map Definition
What does thematic map mean? A thematic map can be defined as a specifically themed map for a particular geographic area. Thematic maps are maps that tend to be about specific topics. The subject matter and the location are determined in the mapmaking process and are required for making all thematic maps. Cartography is the act of mapmaking. Relating data to a geographic location is the process that turns statistical data into geospatial data. We use geospatial every day. Weather maps are a good example. They form a relationship between the weather data, the statistical data, and a location, which becomes the base map.
What Is a Thematic Map Used for?
The purpose of a thematic map is to explain something. Unlike traditional reference maps of a specific place that provide directions or a simple location, thematic maps are explanatory. They do so by depicting data with a location to identify spatial patterns and trends. Their representation of correlated variables visually allows for the study of characteristics of a location, which can lead to a better understanding of a concept. These correlations and understandings can be practiced using the world population map below. For example, why are the lower population areas lower in population? If one is familiar with the planet's physical geography, Siberia, the Amazon Basin, the Sahara, the Himalayas, and the Australian Outback can be identified as extreme environments that few people live.
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There are several different types of thematic maps. These types include isoline maps, cartogram maps, choropleth maps, graduated symbol maps, heat maps, dot-density maps, and flow-line definition maps. Each type of thematic map is used for different purposes, and there are pros and cons for using each. The variety of thematic map types are explored below, including an example of a thematic map of the United States.
Isoline Map Definition
What is an isoline map? Isoline maps use continuous lines that delineate data of the same value. Examples of isoline maps are elevation maps and weather maps. The continual lines depict elevation changes in a map, a specific elevation on one side and another on the other. These elevations will be labeled. Weather maps will use lines and colors to show pressure systems and areas of like temperatures. The benefit of using an isoline map is that it is easy to read and interpret the gradual changes and areas of equal value. The cons are that isoline maps can be challenging to make, and depending on the dataset, they can also be hard to read.
Cartogram Map Definition
A cartogram is a particularly unique type of thematic map. It takes the represented data and stretches or shrinks the data's location on the map. In other words, a cartogram does not represent real physical space. An example of cartograms is population maps that make countries with higher populations bigger and low population countries smaller in size but still in the approximately correct location on the map. Another example would be to show the specific influence of spatial data by using size to indicate domination of specific location data. The benefit to using a cartogram is that it can embody and exhibit numbers well, but a drawback is that it does distort a geographical area.
Choropleth Map Definition
Choropleth maps are one of the most common types of thematic maps. A choropleth map is known for shading colors within the geographic extent of the data to highlight patterns. Often values such as percent, density, or quantity are displayed. An example of a choropleth map would be shading in counties based on political affiliation or population density of countries of the world. A bivariate choropleth map can display two variables at one time. The pros to a choropleth map are that they are easy to read, visually effective, and use geographically common regions and countries. The cons are that the coloring of an area assumes that the area has the same value and does not allow for variations or the ability to get more specific than what is presented.
Graduated Symbol Map Definition
A graduated symbol map uses the size and shape to indicate the amount of the data. This symbol is placed over the location it represents on the map. The larger or smaller the symbol is, the greater or lesser the concentration of data. The symbol is often selected in a shape, form, or color that increases the readability of the map and its data. For example, a circle might represent a specific state, and the circle itself can be a pie chart illustrating something like voting data. Another example would be using the street sign symbol for a hospital, which is an uppercase H; the larger the H, the more hospitals there would be in a specific county.
The advantage of using a graduated symbol map is classifying the symbol proportion to what the data is more effective. This type of symbology can help readers understand the map easier. A disadvantage of this type of map is the size of the symbol can be challenging to relate to an exact location instead of connecting it to a general area.
Heat Map Definition
A heat map depicts the density of a specific phenomenon or feature over its related location. Shading is used to indicate areas of greater or lesser density. This mapping type allows the cartographer to overlay data on a region and not a highly specific location. Heat maps are common in business analysis and the sciences, such as the density of traffic and drought conditions. Pros of heat maps include the use for mapping data without common borders (state or county lines), the opportunity to visualize conditions across borders, and the ease of spotting patterns. The cons of a heat map are that the data is generalized, and any use of color needs to be very specific to be most readable.
Dot Distribution Map Definition
A dot distribution map utilizes a dot or multiple dots to explain the data. Often also called a dot density map, the density of dot locations tends to express the spatial pattern in the data. Dot distribution maps can be used in many ways. Ideas include:
- To plot all disease or virus outbreaks to locate the areas of highest infection and areas of origin.
- To examine how well an ad campaign is being received by mapping the location of each person who looks at it.
- To examine the distribution of people of different levels of wealth to see where the wealthiest and the poorest people reside.
The pros of using dot distribution maps are the option to use simpler data, such as raw data counts and ratios, and the use of black and white maps can be as valuable in explaining the map data as color maps. The cons are they can be laborious to count dots and get specific data information, and to be able to pinpoint the exact location of the data on the map.
Flow-Line Map Definition
Flow-line maps are used to illustrate movement between different locations and regions. They can be as specific as particular latitude and longitude coordinates or as general as a state or region. Flow-lines maps can map the movement of goods, ideas, people, and phenomena, such as weather or the migration of people. The physical flow-line(s) on the map is usually sized to represent the amount of data. The advantages of using a flow-line map are demonstrating movement within a static image and scaling the lines proportionally to the data. The disadvantage of this type of map is that there will be some distortion of distance and direction to overlay these lines on their related locations.
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This thematic map of USA is a choropleth map of the 2016 population of the United States, by state. It is shaded with a purple-blue spectrum - the darker the color, the higher the population. The lighter the color, the smaller the population. Its purpose is to illustrate the most and least populated states visually. This data encourages the map reader to interpret and analyze the information. Why is Florida more populated than North Dakota? What conclusions can the map user draw from this data?
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This lesson focuses on the thematic map and its many uses. A thematic map provides data to explain a phenomenon in a particular location. Cartography is the act of making maps, including thematic maps, which use geospatial data to identify patterns and relationships between a specific topic and the physical location it represents. There are seven common types of thematic maps and the following list of what makes them distinct:
- Isoline map - uses continuous lines to represent data, such as elevation changes.
- Cartogram map - does not represent real physical space; instead, it stretches or shrinks data over its location. Countries distorted to be larger have bigger populations, and the shrunken countries have smaller populations.
- Choropleth map - uses color and shade to represent amounts of data, such as county political affiliation.
- Graduated symbol map - uses size and shape to represent the amount of data, such as larger circles means larger size cities and smaller circles indicate smaller sized cities.
- Heat map - uses shaded areas to represent density over typical borders, such as a weather map.
- Dot distribution map - uses tiny dots to represent each piece of data, such as incidents of disease in a neighborhood.
- Flow-line map - uses a directional line, proportionally sized to the data, such as a shipment of goods from one city to another and in what quantities.
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Video Transcript
Thematic Maps
In the modern world of GPS and smartphones, many of us have forgotten how to appreciate a good old-fashioned map. Maps are awesome. They're full of information and provide such a specific and distinctive understanding of physical space and the people who live there. In fact, maps are amongst the primary ways that we categorize and analyze information about how people interact with and within physical space. Of particular use are thematic maps, those which are designed to illustrate a specific theme in relation to a geographic area. Whether it's trade patterns, or the spread of disease, or even the definitive ranking of people's favorite flavor of potato chips by state, thematic maps always have something fascinating to share.
Choropleth Mapping
Let's look at some of the common ways we create thematic maps. One frequently used system is choropleth mapping, in which statistical data is plotted over predefined geographic areas by coloring them in. For example, if I were to gather statistics on the frequency of bicycle use by state, I've got data that is sorted by a predefined geographical unit, in this case US states, so I could make a choropleth map. Generally, this is done by picking a color, say green, and having different shades represent different values. A state that frequently uses bicycles may be a solid, dark green, while a state that rarely bikes is a light, bright green. That would be a choropleth map.
Isoline Maps
Choropleth maps are fun, but that's not always the best way to map information. If you've got data that is not related to predefined geographic borders and instead focuses on continuity over distance, you might want to consider an isoline map. Isoline maps use continuous lines to indicate points of the same value. You've actually probably seen these before. If you've ever read a map where the landscape is represented as lines of elevation, that's an isoline map. Another common example can be found on the nightly news during the weather segment. When the weather map is divided into lines of pressure, often used to show storm systems, points of the same value are being connected as continuous lines, so it's an isoline map.
Dot Density Map
For another type of thematic mapping, let's turn to dot density mapping, in which dots are used to indicate the presence of a feature. Imagine a map of the United States, now place a dot on it showing where you are. That's a dot density map. You are the only data point on that map, showing us that you are unique. Now, place a dot on the map for every person in the United States of your same gender, or your education level, or your income, or your last name. The distributions of dots over space help researchers identify patterns, especially when you don't necessarily know what you're looking for. These commonly are used to map the spread of disease as a way to locate the source of the disease or areas of high infection.
Flow-Line Maps
Sometimes mapping helps conduct research, as in dot-density maps, and sometimes it's more useful for helping us analyze our research. A flow-line map is a mixture of a map and a flow chart that plots movement across physical space. Basically, it's a map that shows where people or objects came from and where they went. Often, arrows are used to indicate the direction of movement, and the thickness of lines indicates a higher or lower number of people or objects moving. Maps of the history of migration commonly use this. We can see where people move from, where most of them ended up, and roughly how many went to each destination.
Cartograms
A cartogram is a map in which the variable being mapped replaces actual geographic space. Let me explain. In terms of actual physical space, the United States of America is something like 3 million square miles. However, the population is not distributed evenly across those 3 million square miles. Most estimates claim that about 2/3 of Americans live along a coast: the Pacific, Atlantic, or Gulf of Mexico. Now, we could plot this out with a dot density map to show areas of highest population, or we could change the size of the coastal regions to reflect their importance to this data set, thus creating a cartogram.
In this cartogram, the coasts of the United States would be much larger than the area comprising the central, land-locked states, despite the fact that in reality the opposite is true. The point of a cartogram isn't to reflect accurate physical space, it's to present space in a different perspective, more along the lines of what that space means to us. The coasts dominate much of American culture, from New York to Los Angeles, so on our cartogram these regions are exaggerated in size, and the sizeable state of Wyoming, which is not as often featured on American TV shows, is reduced. This map doesn't show us American geography, but it does show us something about American culture.
Lesson Summary
There are many ways you can plot data over a map, depending on your research needs. A thematic map is one which is designed to illustrate a specific theme in relation to a geographic area. There are a few basic kinds. A choropleth map plots statistical data over predefined geographic areas, like states or countries, by coloring them in. An isoline map uses continuous lines to indicate points of the same value and is commonly used for weather or topography. Dot density mapping, in which dots are used to indicate the presence of a feature, is commonly used to show the distribution of data. A flow-line map is a mixture of a map and flow chart that plots movement across physical space. Finally, a cartogram is a map in which the variable being mapped replaces actual geographic space. See, maps are fun. Take that, GPS.
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