A thematic map is a broad category of maps designed to illustrate a specific theme across a geographic area, while a choropleth map is a particular type of thematic map that displays statistical data by coloring or shading predefined geographic regions.
Essentially, the relationship is one of category (thematic map) and sub-type (choropleth map).
Understanding Thematic Maps
A thematic map is one which is designed to illustrate a specific theme in relation to a geographic area. Unlike general reference maps that show locations and features like roads or rivers, a thematic map focuses on a single subject. This theme could be anything from population density, average income, or election results to rainfall patterns, vegetation types, or the distribution of diseases.
There are a few basic kinds of thematic maps, each using different visual methods to represent the theme:
- Choropleth maps: (Discussed below)
- Isoline maps: Use lines connecting points of equal value (e.g., contour lines for elevation, isobars for atmospheric pressure).
- Dot distribution maps: Use dots to show the presence and quantity of a feature (e.g., one dot represents a certain number of people).
- Proportional symbol maps: Use symbols (like circles or squares) whose size is proportional to the value they represent at a specific location (e.g., larger circle indicates more production).
- Cartograms: Distort the area or shape of geographic units to reflect the value of the theme.
Understanding Choropleth Maps
A choropleth map is a very common kind of thematic map. A choropleth map plots statistical data over predefined geographic areas, like states or countries, by coloring them in. The color or shading applied to each area corresponds to the value of the statistical data for that region. Typically, darker or more intense colors represent higher values, while lighter colors represent lower values.
Examples of data often shown on choropleth maps include:
- Population density per square mile by county
- Percentage of the population with a college degree by state
- Median household income by census tract
- Vote share for a political candidate by electoral district
The Key Difference: Category vs. Type
The fundamental difference lies in their scope:
- Thematic Map: A broad category encompassing any map that shows a specific theme.
- Choropleth Map: A specific technique or type within the thematic map category, characterized by using color/shading within defined boundaries to show statistical data intensity.
Therefore, all choropleth maps are thematic maps because they illustrate a theme (statistical data), but not all thematic maps are choropleth maps (e.g., an isoline map showing temperature is thematic but not choropleth).
Comparing Thematic and Choropleth Maps
Here's a quick comparison:
| Feature | Thematic Map | Choropleth Map |
|---|---|---|
| Definition | Illustrates a specific theme | Plots statistical data by coloring predefined areas |
| Relationship | Broad category | Specific type of thematic map |
| Representation | Various methods (color, symbols, lines, dots, etc.) | Primarily color or shading intensity |
| Data Type | Can show various types of data (quantitative, qualitative, distribution) | Best suited for showing intensity or density of statistical data |
| Geographic Basis | Can relate theme to any geography | Uses predefined political or administrative boundaries |
| Examples | Rainfall map, road map, temperature map, population dot map | Population density map, income level map, election results map |
Practical Insights
Choosing the right type of thematic map depends on the data and the message you want to convey. Choropleth maps are excellent for showing how a statistical value varies across standard administrative regions and illustrating patterns of intensity. However, they can sometimes give a misleading impression if the size of the area is disproportionate to the population or phenomenon being measured. Other thematic map types might be better suited for showing absolute counts (proportional symbol maps) or precise point locations (dot maps).
