The Mercator projection has become a standard across web mapping platforms, but has long been considered inappropriate for global data display due to its distortion of high latitude areas. With the ever-rising popularity of web maps, the Mercator projection has seen a resurgence in its use for spatial data visualizations. In this study we investigated the implications of the area distortion effects of the Mercator projection for public data interpretation. We recruited 120 participants via Amazon's Mechanical Turk platform to complete an online survey assessing their ability to identify and account for the distortion effects. Participants were asked to estimate the areas covered by five colored regions on a global map, having been split into a control group using an equal-area projection and a treatment group using a Mercator projection. On average, participants did not discount for the projection and their data interpretation differed between the two conditions as a result. Our findings provide an empirical basis for the distortion effects of the Mercator projection currently used in web maps, and further implicate its appropriateness for displaying global data. More broadly, they introduce experimental methods for research exploring cartographic biases in non-expert groups.
Web map visualization describes the
interactive display of geographic
information on a computer-based map
As a result, there is a growing adoption of
web mapping applications, such as Google
Maps, OpenLayers and Mapbox APIs, in
public data portals and interactive maps
The Mercator projection has become the
standard across web mapping applications
While mapping platforms provide a
powerful and convenient tool for data
visualization, past research has shown that
even experienced users can struggle to
compensate for distortions when making
on-the-spot judgements
For this reason, the Mercator projection has
long been renounced for use in scientific
visualization on the grounds that its area
distortions mislead map readers
A recent body of research has begun to
explore these implications. Notably,
This study assesses the influence of the Mercator projection on area estimation and data interpretation in non-expert audiences. Specifically, we were interested in whether people identify the distortions, and if they do, how able they are to account for them. This question was addressed through an online experimental survey exploring impacts on area-based judgements about global geospatial data. To this end, we advanced two hypotheses: (1) individuals making on-the-fly judgements about spatial data presented on a map are unlikely to identify or correct for projection distortions and; (2) even if individuals are aware of the distortions, they will struggle to accurately convert back to the corresponding areas.
Participants (N = 120) were recruited using
Amazon’s Mechanical Turk online hiring
platform
Participants were randomly assigned to one of two conditions: a treatment condition using a Mercator version of the map (N = 60) and a control condition using an equalarea (Lambert cylindrical) version (N = 53). The control map projection was chosen because areas could be compared at facevalue across the image, while also being a commonly used projection (Šavrič et al., 2015).
The data used in the map visualizations was
derived from a global temperature dataset
downloaded from the University of East
Anglia Climatic Research Unit’s website
To investigate the effects of projection choice on data interpretation, we designed an area estimation and threat perception task. Participants were shown a global-scale map with categorical data displayed (Figure 1.0), which they were told represented the presence of five different pollutants over the earth’s surface. This construction corresponded closely enough to a relatable real-world example, but was abstract enough for participants to engage without strong prior perceptions influencing their responses (a common problem encountered during our pilot surveys which used a temperature labelling scheme). Participants were asked to estimate the total area covered by each of the five pollutants. Further interpretation of the data was evaluated by asking respondents to choose which of two particular colored pollutants they perceived to be a greater threat to the earth.
Participants were next given a short explanation of how different projections unavoidably distort areas and/or shapes displayed on maps. After this briefing, it was hoped that some participants would decide that their previous area judgements had been be influenced by the projection they had been given. They were then shown a blank version of both projections and asked which one they thought was more suitable for an area estimation task, and given the option to alter their original estimates in light of the briefing. Participants in the treatment condition changing their estimations would provide evidence that they had identified and attempted to account for the Mercator Effect.
We tested for the effects of projection type using independent-samples t-tests to compare the equal-area and Mercator conditions across the five area estimations made by participants. We found a significant difference across all the regions. Specifically, participants overestimated the areas which had been enlarged by the Mercator projection, in line with face-value area judgements, as shown in Table 1.0. Similarly, the control condition estimates corresponded closely with the face-value measurements for the equal-area projection. Surprisingly, the answers to the second area estimation question did not differ significantly from the original answers; while some participants in both categories chose to alter their answers, most stuck with their original estimates.
A chi-square test of independence was used to examine the relationship between data interpretation and projection choice. The difference between conditions was significant, 2 (1, N = 113) = 13.58, p < 0.01. In particular, 17% of participants in the equal-area condition (N = 53) perceived pollutant A to be a greater threat than pollutant E, compared to 50% in the Mercator condition (N = 60). A chi-square test was used to test for differences in the answers to the map suitability questions. No significant difference was found between conditions; participants did not judge one projection to be better than the other for making area judgements.
The results from the area estimation and data interpretation tasks indicated that participants’ judgements were significantly affected by the choice of projection. Specifically, participants took the maps at face-value and interpreted the data accordingly. This result was corroborated by responses to follow-up questions, which suggested that participants identified the Mercator projection as being equally appropriate to the control projection for area estimation tasks, as well as the fact that they chose not to adjust their answers to the second part of the survey.
Further work would be necessary to refine
the methods used in this study. It is possible
that some of the documented effects could
have been observed if participants had not
fully understood the wording of the
questions. Additionally, there were several
unaddressed confounds between the two
conditions which could have contributed
towards the observed differences, such as
the image dimensions and differences in
granularity between the maps which arose
due to scaling deformations. Despite these
limitations, the central result, that the
Mercator projection biases global data
interpretation, has concrete implications for
geovisualization and GIScience research.
This study has provided empirical evidence
for the Mercator Effect in web maps. We
found that individuals were unlikely or
unable to identify and re-project area data
displayed on a Mercator projection to
corresponding areas on the earth’s surface,
corroborating past research
This study was completed in the Department of Geography at McGill University under the supervision of Professor Renee Sieber and Dr. Jin Xing, both of whom I thank for their support and guidance. We also thank the two anonymous reviewers for their helpful comments on earlier drafts of the manuscript.