Cartogram

General principles

Since the early days of the academic study of cartograms, they have been compared to map projections in many ways, in that both methods transform (and thus distort) space itself. The goal of designing a cartogram or a map projection is therefore to represent one or more aspects of geographic phenomena as accurately as possible, while minimizing the collateral damage of distortion in other aspects. In the case of cartograms, by scaling features to have a size proportional to a variable other than their actual size, the danger is that the features will be distorted to the degree that they are no longer recognizable to map readers, making them less useful.

As with map projections, the tradeoffs inherent in cartograms have led to a wide variety of strategies, including manual methods and dozens of computer algorithms that produce very different results from the same source data. The quality of each type of cartogram is typically judged on how accurately it scales each feature, as well as on how (and how well) it attempts to preserve some form of recognizability in the features, usually in two aspects: shape and topological relationship (i.e., retained adjacency of neighboring features). It is likely impossible to preserve both of these, so some cartogram methods attempt to preserve one at the expense of the other, some attempt a compromise solution of balancing the distortion of both, and other methods do not attempt to preserve either one, sacrificing all recognizability to achieve another goal.

Area cartograms

The area cartogram is by far the most common form; it scales a set of region features, usually administrative districts such as counties or countries, such that the area of each district is directly proportional to a given variable. Usually this variable represents the total count or amount of something, such as total population, gross domestic product, or the number of retail outlets of a given brand or type. Other strictly positive ratio variables can also be used, such as GDP per capita or birth rate, but these can sometimes produce misleading results because of the natural tendency to interpret size as total amount. Of these, total population is probably the most common variable, sometimes called an isodemographic map.

The various strategies and algorithms have been classified a number of ways, generally according to their strategies with respect to preserving shape and topology. Those that preserve shape are sometimes called equiform, although isomorphic (same-shape) or homomorphic (similar-shape) may be better terms. Three broad categories are widely accepted: contiguous (preserve topology, distort shape), non-contiguous (preserve shape, distort topology), and diagrammatic (distort both). Recently, more thorough taxonomies by Nusrat and Kobourov, Markowska, and others have built on this basic framework in an attempt to capture the variety in approaches that have been proposed and in the appearances of the results. The various taxonomies tend to agree on the following general types of area cartograms.

Anamorphic Projection

This is a type of contiguous cartogram that uses a single parametric mathematical formula (such as a polynomial curved surface) to distort space itself to equalize the spatial distribution of the chosen variable, rather than distorting the individual features. Because of this distinction, some have preferred to call the result a pseudo-cartogram. Tobler's first computer cartogram algorithm was based on this strategy, for which he developed the general mathematical construct on which his and subsequent algorithms are based. This approach first models the distribution of the chosen variable as a continuous density function (usually using a least squares fitting), then uses the inverse of that function to adjust the space such that the density is equalized. The Gastner-Newman algorithm, one of the most popular tools used today, is a more advanced version of this approach. Because they do not directly scale the districts, there is no guarantee that the area of each district is exactly equal to its value.

Shape-warping contiguous cartograms

Also called irregular cartograms or deformation cartograms, This is a family of very different algorithms that scale and deform the shape of each district while maintaining adjacent edges. This approach has its roots in the early 20th Century cartograms of Haack and Weichel and others, although these were rarely as mathematically precise as current computerized versions. The variety of approaches that have been proposed include cellular automata, quadtree partitions, cartographic generalization, medial axes, spring-like forces, and simulations of inflation and deflation. Some attempt to preserve some semblance of the original shape (and may thus be termed homomorphic), but these are often more complex and slower algorithms than those that severely distort shape.

Non-contiguous isomorphic cartograms

This is perhaps the simplest method for constructing a cartogram, in which each district is simply reduced or enlarged in size according to the variable without altering its shape at all. In most cases, a second step adjusts the location of each shape to reduce gaps and overlaps between the shapes, but their boundaries are not actually adjacent. While the preservation of shape is a prime advantage of this approach, the results often have a haphazard appearance because the individual districts do not fit together well.

Diagrammatic (Dorling) cartograms

In this approach, each district is replaced with a simple geometric shape of proportional size. Thus, the original shape is completely eliminated, and contiguity may be retained in a limited form or not at all. Although they are usually referred to as Dorling cartograms after Daniel Dorling's 1996 algorithm first facilitated their construction, these are actually the original form of cartogram, dating back to Levasseur (1876) and Raisz (1934). Several options are available for the geometric shapes:

• Circles (Dorling), typically brought together to be touching and arranged to retain some semblance of the overall shape of the original space. These often look like proportional symbol maps, and some consider them to be a hybrid between the two types of thematic map.
• Squares (Levasseur/Demers), treated in much the same way as the circles, although they do not generally fit together as simply.
• Rectangles (Raisz), in which the height and width of each rectangular district is adjusted to fit within an overall shape. The result looks much like a treemap diagram, although the latter is generally sorted by size rather than geography. These are often contiguous, although the contiguity may be illusory because many of the districts that are adjacent in the map may not be the same as those that are adjacent in reality.

Because the districts are not at all recognizable, this approach is most useful and popular for situations in which the shapes would not be familiar to map readers anyway (e.g., U.K. parliamentary constituencies) or where the districts are so familiar to map readers that their general distribution is sufficient information to recognize them (e.g., countries of the world). Typically, this method is used when it is more important for readers to ascertain the overall geographic pattern than to identify particular districts; if identification is needed, the individual geometric shapes are often labeled.

Mosaic cartograms

In this approach (also called block or regular cartograms), each shape is not just scaled or warped, but is reconstructed from a discrete tessellation of space, usually into squares or hexagons. Each cell of the tessellation represents a constant value of the variable (e.g., 5000 residents), so the number of whole cells to be occupied can be calculated (although rounding error often means that the final area is not exactly proportional to the variable). Then a shape is assembled from those cells, usually with some attempt to retain the original shape, including salient features such as panhandles that aid recognition (for example, Long Island and Cape Cod are often exaggerated). Thus, these cartograms are usually homomorphic and at least partially contiguous.

This method works best with variables that are already measured as a relatively low-valued integer, enabling a one-to-one match with the cells. This has made them very popular for visualizing the United States Electoral College that determines the election of the president, appearing on television coverage and numerous vote-tracking websites. Several examples of block cartograms were published during the 2016 U.S. presidential election season by The Washington Post, the FiveThirtyEight blog, and the Wall Street Journal, among others. This is a cartogram for the 2024 and 2028 elections, based on the 2020 Census apportionment:

The major disadvantage of this type of cartogram has traditionally been that they had to be constructed manually, but recently algorithms have been developed to automatically generate both square and hexagonal mosaic cartograms.

Linear cartograms

A linear cartogram manipulates the travel distance (i.e., acummulated geographic distance) along a linear feature. The spatial distortion allows the map reader to easily visualize intangible concepts such as travel time and connectivity on a network. Distance cartograms are also useful for comparing such concepts among different geographic features. A distance cartogram may also be called a central-point cartogram.

A common use of distance cartograms is to show the relative travel times and directions from vertices in a network. For example, on a distance cartogram showing travel time between cities, the less time required to get from one city to another, the shorter the distance on the cartogram will be. When it takes a longer time to travel between two cities, they will be shown as further apart in the cartogram, even if they are physically close together.

Distance cartograms are also used to show connectivity. This is common on subway and metro maps, where stations and stops are shown as being the same distance apart on the map even though the true distance varies. Though the exact time and distance from one location to another is distorted, these cartograms are still useful for travel and analysis.

Multivariate cartograms

Both area and linear cartograms adjust the base geometry of the map, but neither has any requirements for how each feature is symbolized. This means that symbology can be used to represent a second variable using a different type of thematic mapping technique. For linear cartograms, line width can be scaled as a flow map to represent a variable such as traffic volume. For area cartograms, it is very common to fill each district with a color as a choropleth map. For example, Worldmapper has used this technique to map topics relating to global social issues, such as poverty or malnutrition; a cartogram based on total population is combined with a choropleth of a socioeconomic variable, giving readers a clear visualization of the number of people living in underprivileged conditions.

Another option for diagrammatic cartograms is to subdivide the shapes as charts (commonly a pie chart), in the same fashion often done with proportional symbol maps. This can be very effective for showing complex variables such as population composition, but can be overwhelming if there are a large number of symbols or if the individual symbols are very small.

Production

One of the first cartographers to generate cartograms with the aid of computer visualization was Waldo Tobler of UC Santa Barbara in the 1960s. Prior to Tobler's work, cartograms were created by hand (as they occasionally still are). The National Center for Geographic Information and Analysis located on the UCSB campus maintains an online Cartogram Central Archived 2016-10-05 at the Wayback Machine with resources regarding cartograms.

A number of software packages generate cartograms. Most of the available cartogram generation tools work in conjunction with other GIS software tools as add-ons or independently produce cartographic outputs from GIS data formatted to work with commonly used GIS products. Examples of cartogram software include ScapeToad, Cart, and the Cartogram Processing Tool (an ArcScript for ESRI's ArcGIS), which all use the Gastner-Newman algorithm. An alternative algorithm, Carto3F, is also implemented as an independent program for non-commercial use on Windows platforms. This program also provides an optimization to the original Dougenik rubber-sheet algorithm. The CRAN package recmap provides an implementation of a rectangular cartogram algorithm.

Algorithms