Cartograms,hexograms and regular grids: Minimising misrepresentation in spatial data visualisations

Abstract

The visualisation of spatial information is a powerful tool for researchers in urban analytics seeking to convey their findings to the wider research community and the public in an accessible way. Yet, even the most well-intentioned cartographer may introduce misrepresentation by mapping irregularly shaped and sized areas. This paper explores the extent to which different methods of visualising area-based data can remedy (or exacerbate) this misrepresentation by presenting results from a crowdsourced survey. Data from the 2016 European Union referendum at Local Authority level in England are visualised using four alternative methods (balanced cartogram, hexogram, hexagonal grid, square grid) and compared to a traditional choropleth map, in terms of people’s understanding of the authors’ intended message, through a crowdsourced survey questionnaire. Results indicate that mapping out original boundaries can introduce misrepresentation, which can be mitigated by using balanced cartograms and hexograms to improve the accuracy of visualisations.

Keywords

Visualisation cartogram hexogram Brexit

Introduction

Thematic maps are powerful, accessible and aesthetically appealing visualisations widely applied to represent spatial data (Barrozo et al., 2016). In urban analytics, spatial data visualisation is important to effectively communicate and engage with stakeholders (Billger et al., 2017) and can even serve to analyse geographical information (Rae, 2011). However, irregularly shaped polygons and large differences in the sizes of areas being mapped can introduce misrepresentation. The message researchers want to get across might be lost, or misunderstood by readers. To address this issue, methods have been developed to distort the shape and size of areas, either by turning irregular polygons (such as neighbourhoods) into regular or hexagonal grids (Bailey, 2018), or by using cartograms, where the distortions of size and shape are made explicit and communicate meaning (Dorling, 1996; Tobler, 2004). Such methods have been used to illustrate the spatial distribution of variables across numerous fields at a variety of spatial scales, from population health in Japan (Nakaya, 2010) to regional ‘misery’ across Europe (Pontarollo et al., 2018).

That said, it is unclear how these different transformations can impact on viewers’ interpretation of the map. Using a crowdsourced survey, we explore the extent to which alternative methods of visualising spatial data can improve communication of an intended message by testing people’s understating of maps transformed using four different contemporary methods. This is motivated by existing calls for geographers to ‘gain a better understanding of the map communication process’ by examining how accurately people interpret different methods of spatial visualisation (Lloyd and Steinke, 1977: 436). We hope that these findings highlight the issue of misrepresentation in spatial data for the urban analytics community, but more specifically, we aim to provide some guidance as to which methods might the most appropriate.

Background

There has been an interest in the way in which people read maps since early studies on the perception of orientation and imaginary maps (Gulliver, 1908 and Trowbridge, 1913 cited in Mark et al., 1999). One landmark study by Flannery (1971) focused on the distortions in perceived sizes of cartographic symbols, using laboratory experiments to measure perceptual bias (Mark et al., 1999). From this work emerged a consensus that ‘[m]aps must provide accurate information to be useful, but they also must have an understandable message and be aesthetically pleasing’, which guided much research in cartography (Mark et al., 1999: 752). In urban analytics, thematic maps have become an increasingly common way to represent data (Barrozo et al., 2016), and research has begun to consider how viewers draw meaning from such maps.

There are many choices to be made when creating a thematic map which can affect people’s understanding of the information being presented. For instance, in the case of a quantitative variable, the decision about whether the variable is quantised or presented as continuous can impact upon how viewers interpret the data (Muller, 1980). For a quantised map, one must decide which choropleth classification method is most suitable to present the variable of interest. The classification used and the continuous shading implemented can dictate people’s ability to accurately understand the visualisation and the message being conveyed (Brewer and Pickle, 2002; Lloyd and Steinke, 1976). In terms of topology, it is also possible to introduce deliberate area distortions to the visualisation, to address a specific problem common to traditional area-based choropleth maps, whereby variation in the size and shape of areas being visualised affects map legibility (Dorling, 1996; Stigmar and Harrie, 2011).

In extreme cases of this, larger areas come to dominate the map and render smaller regions almost invisible. Census data in England and Wales, for instance, are published at spatial scales designed to be uniform by population (e.g. Lower Super Output Area, a census block unit in England and Wales). Consequently, sparsely populated areas dominate visualisations at the expense of those that are densely populated. In such circumstances, even the most well-intentioned researcher, using geographically accurate spatial data, may introduce a degree of misrepresentation in their visualisations or fail to communicate their message to readers as intended.

To date, a popular method for overcoming these obstacles has been the cartogram. Although there are numerous methods of operationalising cartograms (Dougenik et al., 1985), the underlying premise is that areas are rescaled according to a variable (Nusrat and Kobourov, 2016). By rescaling areas by some uniform variable (such as population, in the example of Lower Super Output Areas), an effort is made to minimise the misrepresentation that can be introduced by using raw area boundaries. Larger areas become smaller, and less dominant, and ‘invisible’ areas are expanded to become more visible. This approach has come under some criticism for alleviating mispresentation through invisibility at the expense of introducing misrepresentation through distortion (Harris et al., 2017a). Even well-specified scaling variables can cause alterations which result in some polygons appearing as lines, for instance (Coltekin, 2016).

A recent development is the ‘balanced area’ cartogram, which aims to minimise the distorting side-effects of cartograms (see Harris et al., 2017a, 2017b). The balance is achieved by predefining an ‘interpretability threshold’ which is the smallest legible unit size given the dimensions of the final published map. In producing the cartogram, any areas that fall below this areal threshold are ‘protected’ from the rescale, and instead are set as the minimum unit size. Harris demonstrated the benefits of this approach using Local Authority data on residential geography in England. The degree of error, defined as the percentage of non-overlap between the original map and the cartogram, was minimised with the balanced cartogram compared to a solely attribute-scaled (e.g. population) cartogram (Harris, 2017).

This approach has also been extended to include a ‘hexogram’, whereby an iterative binning algorithm assigns the centroid of polygons from the balanced cartogram to tessellated hexagons, each representing the original polygons. In doing so, the data are said to maintain spatial accuracy whilst also being uniform in shape and size (Harris et al., 2018a, 2018b). Comparable alternatives to this approach are tile maps which use a distance-based procedure (e.g. Hungarian algorithm) to assign original polygons to a grid of uniform shapes, such as hexagons or squares, in a manner that minimises the distance between the original and the new synthetic boundaries (Bailey, 2018). In doing so, tile maps generate an aesthetically appealing contiguous grid of polygons which can introduce topological inaccuracies, such as previously separated polygons becoming neighbours. The hexogram prioritises the maintenance of the original topological links, but it is not contiguous. In each case, the stylised map retains the same number of observations as the original map, but the boundaries have been transformed into something more uniform and less distracting, which may be better suited for conveying the message of the researcher.

Currently, little is known about how these different methods of visualising spatial data impact on people’s interpretation of the information presented. But as seen from the development of research into people’s understanding of maps, there is a need to empirically test the principles of visual-display design (Hegarty, 2013). This study aims to rectify this shortcoming through the use of a crowdsourced online survey questionnaire designed to measure the extent to which various alternatives to a traditional thematic map can more accurately convey geographic information. In what follows, we provide an outline of the survey design and methodology, followed by the reporting of results, and conclude with a discussion on our findings and suggestions for future research.

Survey design

Studies have made some attempt to gauge how people interpret different visualisations of the same data to draw conclusions (e.g. Borgo et al., 2012; Borkin et al., 2016; Skau and Kosara, 2016). Specific to maps, researchers have been using survey designs to examine how people interpret spatial information and visualisation techniques for some time (see McCarty and Salisbury, 1961). For instance, studies examining the impact of different ways of shading choropleth maps have used survey samples of undergraduate geography students (Lloyd and Steinke, 1976). Brewer and Pickle’s (2002) study involved giving undergraduate students map-reading questions to determine which classification method helped them best interpret the spatial information presented. Coltekin et al. (2015) asked respondents to complete a series of tasks using various different tools available in Google Maps (e.g. 2D default map, 3D satellite images, Street View) and found that the degree of accuracy with which people completed questions varied by the tool used. More recently, ‘visualization researchers have been increasingly leveraging crowdsourcing approaches to overcome a number of limitations of controlled laboratory experiments, including small participant sample sizes and narrow demographic backgrounds of study participants’ (Borgo et al., 2018: 573).

Here, we use a crowdsourced survey to assess the ability of different thematic mapping techniques to visualise and communicate a situation where high values spatially cluster in small areas. Descriptive maps can play an important role in identifying and understanding spatial clusters in urban analytics, despite continued advances in more complex statistical methods (e.g. Jones et al., 2018). We used electoral result data from the 2016 European Union (EU) referendum at Local Authority level in England to create a map considered to be a good example of high value clustering which is obscured by significant differences in area sizes. Areas with a high proportion of Remain votes are concentrated in Greater London (Hobolt, 2016), which has geographically small Local Authorities compared to the rest of the country. On a traditional thematic map, using original boundaries as defined by the Office of National Statistics, strongly Leave areas dominate the visual at the expense of densely populated Remain areas, which became almost ‘invisible’ (see Figure 1).

Figure 1.

Proportion of remain votes in 2016 EU referendum by Local Authority area in England using original boundaries.

Alternatives to this original map were then generated using four different techniques for transforming the Local Authority area polygons. Balanced area-based cartograms¹ and hexograms were created in R (version 3.5.1) using the default minimum threshold options (see Harris, 2017). Uniform hexagonal and square tile grids were generated using the geogrid R package using the default options regarding the optimisation of cell sizes (Bailey, 2018). A decision was made to create the uniform grids from the balanced cartogram rather than from the original boundaries to produce a more optimal outcome and reduce computation time. A result of this was that the outputted boundaries were not completely contiguous, contrary to what was produced using the original boundaries. In total, five visualisations were created: the original (see Figure 1), balanced cartogram, hexogram, hexagonal grid and square grid (see Figure 2). Polygons were shaded according to the percentage of Remain voters in each Local Authority.

Figure 2.

Proportion of Remain votes in 2016 EU referendum by Local Authority area in England using (a) balanced cartogram, (b) hexogram, (c) square grid, (d) hexagonal grid.

These maps were then collated in a survey, and for each map, participants were asked to rate the extent of their agreement with a statement considered to be a true and accurate description of the data, namely: ‘High values (in yellow) appear to be clustered near one another, with a handful of outliers elsewhere in the country’. Respondents viewed the five maps in isolation and reported their agreement with the statement using a 5-point Likert scale (strongly agree, slightly agree, neither agree nor disagree, slightly disagree, strongly disagree). There was an additional option for ‘Don’t know’. If respondents agreed with the statement, the map was considered to be representing the data accurately and conveying the message of high value clustering as intended.

The survey itself did not include any reference to the referendum or the method used to generate each map. Options were shown in the same order for every respondent (starting from the balanced cartogram, hexogram, hexagonal grid, original and finally the square grid). In the interests of keeping the survey as short as possible, and to minimise dropout, no demographic information (e.g. gender, education) was asked of the respondents. The survey was created using Google Forms and distributed via social media platforms Twitter and Reddit (using the r/samplesize subreddit). Possible issues around pre-hoc knowledge and other sample biases are discussed in detail after the results.

Results

The survey was deployed for three days, generating a sample size of 768 respondents, all of which had completed the questionnaire in its entirety. Overall, the map with the highest proportion of respondents agreeing with the statement was the balanced cartogram, with 94% of respondents reporting agreement (either ‘strongly agree’ or ‘slightly agree’). Only two respondents did not know and a total of 4% disagreed with the statement in some way (either ‘strongly disagree’ or ‘slightly disagree’). This was followed by the hexogram, for which 91% of respondents showed some degree of agreement, two respondents did not know and 6% either strongly or slightly disagreed with the statement. Although in general, respondents appeared to interpret the original map as intended, with 77% reporting some level of agreement with the statement, this was still lower than the balanced cartogram and hexogram. The original map also had the highest number of don’t know responses (12). The regular hexagonal and square grids had the highest level of disagreement, with only 53% and 51% either strongly or slightly agreeing with the statement, respectively.

For further analysis, the 5-point Likert scale was recoded to create a binary outcome variable to indicate whether respondents ‘agreed’ (aggregate strongly agreed, slightly agreed) or ‘disagreed’ (aggregate strongly disagreed, slightly disagreed) with the true statement (‘high values (in yellow) appear to be clustered near one another, with a handful of outliers elsewhere in the country’) for use in a logistic regression model. Whilst we acknowledge that the sample is not random, thus compromising generalisability to a wider population, this analysis was considered useful for gauging effect sizes. Respondents who answered ‘Don’t know’ or ‘Neither agree nor disagree’ were excluded from this analysis. The outcome variable (agree–disagree) was regressed on a single categorical variable indicating which map type respondents were observing when answering the Likert scale question, with the original map set as the reference category. The estimates obtained from the model provide an indication as to how likely respondents were to agree with the true statement when viewing each map type. Odds ratio estimates confirm the descriptive findings (see Table 1). The odds of respondents agreeing with the true statement were 3.8 and 2.6 times that of the original map for the balanced cartogram and hexagram, respectively. When viewing the hexagonal or square grids, people were about three times less likely to agree with the statement compared to the reference category of the original map. These odds were significant at p < 0.001 alpha, noting the issue regarding generalisability of these results to a wider population due to self-selection in the crowdsourced sample.

Table 1.

Logistic regression results, response variable recorded from Likert-scale to binary agree (1) disagree (0); original map is reference category.

Map type	B	Se	Z ratio	Probability	Odds
Original (reference)	–	–	–	–	–
Balanced cartogram	1.3364	0.2047	6.529	<0.001	3.80 (CIs: 2.57–5.76)
Hexogram	0.9478	0.1827	5.187	<0.001	2.58 (CIs: 1.81–3.72)
Square grid	−1.3205	0.1320	−10.000	<0.001	0.27 (CIs: 0.21–0.34)
Hexagonal grid	−1.3130	0.1314	−9.989	<0.001	0.27 (CIs: 0.21–0.35)

Note: 95% confidence intervals reported; Likelihood-ratio Model Chi Square: 553.418 (p<0.001); Likelihood-ratio pseudo R2: 0.155; AIC: 3036.5.

Discussion

Overall, findings suggest that it is possible to improve communication of a message by transforming map boundaries, in particular using balanced cartogram and hexogram approaches. However, it is also possible to impair interpretation, as seen with transformations into tiled grids (square and hexagonal), for which fewer people interpreted the visualisation as intended. In reading these results, there are a number of discussion points to consider.

Firstly, the research design assumes that the ‘true’ statement was both an accurate description of the raw data and a fair means of gauging the extent to which respondents were understanding each map. We acknowledge that the statement could have been written differently, with some respondents demonstrating lack of understanding (e.g. those that answered ‘Don’t know’) possibly as a product of the research design, rather than the visualisations themselves. Secondly, due to a lack of individual characteristic data, we were unable to control for other factors that might have impacted on how respondents interpreted the task. For instance, those familiar with cartography, or data visualisation more generally, may have been more sensitive to the issues that can arise when creating maps. The degree to which our sample was representative of a wider population, whether it be the general public or geographers, is unknown. Thirdly, although the survey contained no reference to the EU referendum, respondents familiar with England’s social geography may have drawn upon other personal information to inform their decision. For instance, many other social phenomena (e.g. income) are commonly known to cluster in the urban areas where Remain voters also cluster.

The findings from this survey mark an attempt to empirically examine how people interpret different methods of visualising area-based thematic maps. As such, we highlight some key directions for future research. Firstly, as mentioned above, collecting individual characteristics on respondents could help control for confounding factors such as expertise and assess generalisability of findings. Secondly, questions could be added to test different dimensions of misrepresentation or improve objectivity. For instance, respondents could be asked to manually identify areas of high clustering by clicking or selecting on the map interactively, which could then be compared to the ‘true’ results from a Local Indicators of Spatial Autocorrelation analysis. Thirdly, there are other methods of generating thematic maps that we did not consider. It would be interesting to compare the balanced cartogram to other cartogram algorithms (e.g. Gastner and Newman, 2004) scaled by a selection of variables, or alternatives such as the Dorling cartogram (see Dorling, 2012: 144). Future research could consider making these advancements using synthetic data, to mitigate the impact of prior knowledge of the study area.

Software

For those interested in reproducing these methods in R (R Core Team, 2018) for their own research, the balanced cartogram and hexogram used open source code (see Harris, 2017) which included the packages cartogram (Jeworutzki, 2018), fMultivar (Wuertz et al., 2017) and sp (Pebesma and Bivand, 2005). Spatial data handling and visualisations were carried out using sf (Pebesma, 2018), tidyverse (Wickham, 2017) and viridis (Garnier, 2018).

Footnotes

Acknowledgements

We would like to thank the three anonymous reviewers who provided comments on an earlier version of this article, and in doing so, greatly improved the final version of the paper. We are also extremely grateful to the Quantitative Spatial Science research group at the University of Bristol, especially Richard Harris, for providing feedback on the findings from this study and making suggestions for future research.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Samuel Langton’s research contribution was completed under a Vice Chancellor's doctoral scholarship at Manchester Metropolitan University.

Note

Samuel H Langton is a doctoral student and research assistant focusing on the geographic distribution of crime and offender residences in urban areas. He is also interested in non-crime police demand and data visualisation using open software.

Reka Solymosi is a Lecturer in quantitative methods focused on making use of new forms of data to gain insight into people’s behaviour and subjective experiences, particularly focusing on crime, transport, and spatial research. She is also interested in promoting data literacy.

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