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Thematic map

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A thematic map is a specialized tool used to show geographic patterns of a specific subject. While general reference maps focus on the location of physical features like roads or rivers, thematic maps focus on a theme. This theme represents properties that are not naturally visible to the eye. For example, you cannot see population density or language distribution just by looking at a landscape.

choropleth
choropleth

These maps use symbols to visualize data such as temperature, soil types, or public health statistics. By isolating a single subject, they allow researchers to see how information is spread across an area.

Cartographers use several distinct techniques to represent these data layers. One common method is the choropleth map. This technique uses colors or shading to represent statistical data within predefined regions, such as states or countries.

choropleth
choropleth

A map might use different levels of lightness to show population density. Another method is the proportional symbol map. This uses shapes, like circles or squares, at specific locations. The size of the symbol is proportional to the value it represents, such as the total population of a city.

2016 US Presidential Election Pie Charts
2016 US Presidential Election Pie Charts

A third method is the cartogram. This technique intentionally distorts the size or shape of geographic areas. The distortion is based on a specific variable, making a country appear larger or smaller depending on its data.

PaullHennig2016WorldMap.OAha.CC-BY-4.0
PaullHennig2016WorldMap.OAha.CC-BY-4.0

There are also isarithmic maps, which use lines to connect points of equal value. These are often used to show continuous data like temperature or elevation.

USDA Hardiness zone map
USDA Hardiness zone map

Another important type is the flow map. These maps visualize the movement of items or people between locations.

FlowMap
FlowMap

For instance, a flow map might show the movement of aid from one country to another. Each of these mapping layers can be combined to create more complex bivariate or multivariate maps. These advanced maps allow a viewer to see the relationship between two or more different properties at once.

The history of thematic mapping is closely tied to the Industrial Age. While some roots exist in the Enlightenment, most modern graphical techniques were developed between 1700 and 1850. Before this period, cartography mostly focused on creating accurate base maps. One of the earliest examples was a 1607 map by Jodocus Hondius. This map used symbols to show the dispersion of major religions. Later, the astronomer Edmond Halley contributed significantly to the field. In 1701, he published a chart showing lines of equal magnetic variation.

Halley compass variations 1702
Halley compass variations 1702

This work is considered one of the first isarithmic maps.

During the 19th century, thematic mapping entered a "golden age." In 1826, Charles Dupin created the first known choropleth map. In 1854, the physician John Snow used a thematic map to perform a vital scientific analysis.

Snow-cholera-map-1
Snow-cholera-map-1

He mapped cholera deaths in a London neighborhood. By observing the pattern, he identified a specific water pump as the source of the disease. This methodology anticipated the modern principles of Geographic Information Systems, or GIS. Later, Charles Joseph Minard became a master of the field. He integrated flow maps with statistical charts to create visual narratives.

Minard
Minard

His 1869 map of Napoleon's invasion of Russia remains a famous example.

Today, thematic maps serve several essential purposes in science and daily life. They can provide specific information about a location, such as the proportion of a certain group in a city. They can also show general spatial patterns, like where specific crops are grown. Furthermore, they allow for the comparison of patterns over time, such as changes in election results. The popularity of these maps grew in the late 20th century due to the Quantitative revolution in geography. This movement increased the use of maps as tools for scientific communication.

Modern technology has revolutionized how these maps are produced and used. In the past, every map had to be drafted manually. Now, personal computers, graphics software, and the Internet facilitate rapid design. The rise of GIS allows researchers to handle massive amounts of digital data. For example, the digital release of national censuses in the 1990s provided huge volumes of data for mapping. This connection between technology and data makes thematic mapping a vital part of modern geovisualization and geographic study.

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🖼️ Images & Media (11)
File:Minard.png
Minard.png
File:USDA Hardiness zone map.jpg
USDA Hardiness zone map.jpg
File:Halley compass variations 1702.jpg
Halley compass variations 1702.jpg
File:Snow-cholera-map-1.jpg
Snow-cholera-map-1.jpg
File:choropleth.gif
choropleth.gif
File:2016 US Presidential Election Pie Charts.png
2016 US Presidential Election Pie Charts.png
File:PaullHennig2016WorldMap.OAha.CC-BY-4.0.jpg
PaullHennig2016WorldMap.OAha.CC-BY-4.0.jpg
File:January 17 1982 500-Millibar Height Contours.png
January 17 1982 500-Millibar Height Contours.png
File:Geologic Map of Georgia.png
Geologic Map of Georgia.png
File:Dot Density.png
Dot Density.png
File:FlowMap.png
FlowMap.png
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