A cartogram is called a thematic map in which a mapping variable – such as travel time, population, or GNP – is substituted for land area or distance. As the space or geometry of the map is used to convey the information of an alternative variable, the map is usually distorted and sometimes to a large extent. Cartograms have been used for a long time, primarily to display emphasis and minimize the natural distortions caused by our perception of real geography.
What: Discover Rank
Why: Revisualize geographical entities as per prominence
History of Cartogram
The French engineer Charles Minard is largely credited as the first to use the term cartogram (Friis, 1974). Minard was a pioneer of statistical graphs and charts in the mid-1800s. While maps and atlases had begun to use cartograms of some types, perhaps the earliest example of its use in a general English-speaking publication was in The Washington Post in 1929. Joseph R. Grundy used a cartogram to illustrate his belief that State-based voting powers were unfair as they all had markedly different populations. However, one of the first cartographers to generate cartograms with the aid of computer visualization was Waldo Tobler of UC Santa Barbara in the 1960s. Cartograms can also be constructed manually, either by hand or in a computer-assisted environment. Now a number of software packages generate cartograms.
When to Use a Cartogram
1Represent a population through area cartograms
Use a cartogram when you need to represent an area in the context of the value a variable associated with it holds, making it useful as an isodemographic map. Cartograms are particularly useful as population cartograms, which can illustrate the relative sizes of the populations of the countries of the world by scaling the area of each country in proportion to its population. In such scenarios, the shape and relative location of each country is retained to as large an extent as possible. Cartograms can traditionally show a single data variable at a time but by modifying the color or by using textures or glyphs additional variables could also be incorporated.
2When data representation can do without preservation of shape and topology
One can also use cartograms when shape and topology preservation might be a lower priority than representing values to enable comparisons. In such cases, cartograms like Dorling replaces actual shapes with circles scaled according to the mapped features, where circles are distributed to resemble the original topology. Demers cartogram is a variation of Dorling cartogram, but it uses rectangles instead of circles and also attempts to retain visual cues at the expense of minimum distance. Schematic maps based on quadtrees can be seen as non-shape-preserving cartograms when some degree of neighborhood preservation might be needed.
3Map out distances while visualizing proximities
Use cartograms to visualize the proximities between points, such as travel time. The distance cartogram is appropriate visualization alters distances between a user-specified origin and the other locations in a map with respect to travel time. Using a distance cartogram one can weigh the relative travel time costs between the origin and potential destinations at a glance because travel times are projected in a linearly interpolated time-space from the origin.
Types of Cartograms
1. Contiguous Cartograms
These charts distort the geometry of geographical regions in proportion to the data value associated with that region, such that larger the value, the more distorted and enlarged the region’s area is.
2. Non-Contiguous Cartograms
Invented by Judy M. Olson, they preserve the shape or outline of geographical regions and rescale each region from its center-point, in proportion with the data values assigned to them.
3. Dorling Maps
Here geographical regions are converted into circles and are organized and positioned in a way that loosely resembles the original topology. The area of the circles are proportional to the values they represent.
4. Demers Cartograms
Similar to a Dorling Map, it visualizes data but uses squares instead of circles, reducing the gaps between each geographical region.
5. Mosaic Cartograms
Unlike a Demers Cartogram which uses squares of varying sizes, this Cartogram variation keeps the size of the squares uniform.
6. Cartogram Hexmaps
Similar to mosaic cartograms these charts use hexagons instead of squares. Tilegrams is a tool which can be used to generate these charts.
7. Distance Cartograms
This Cartogram distorts a map (either geographical or an abstract travel network map) to show the travel times from a particular position on that map.
When Not to Use a Cartogram
1When required to make statistically accurate trade-off decisions
Do not use cartograms when balancing statistical accuracy, geographical accuracy and topological accuracy are of importance to you. Maps depend on variation in scale to represent a large area and a cartogram is not a true representation of the real-world area and may give incomplete information. One of the disadvantages of the cartogram is that it inevitably changes the visual representation of geography.
2When interpretation needs to be simple and free of bias
Cartography has allowed maps to show only specific features that are important to users at a specific time and not all the features that are available in the area. Cartograms mostly use symbols to represent objects on land and these require interpretation. Also, since maps are drawn and designed by different entities who may have certain biases towards certain areas, hence full information regarding the area might not be represented.