Use contiguous cartograms instead of rectangular cartograms for area-comparison tasks

Use a contiguous cartogram when readers must compare region areas, identify the largest regions, or judge whether a region grew or shrank. For example, replace a rectangular cartogram with a contiguous cartogram for bigger-than, top-k, or change-detection tasks.

Area judgments get harder when region shapes become overly schematic or visually awkward, and contiguous cartograms reduce that problem relative to rectangular ones.

Mechanism: A contiguous cartogram keeps the region as a connected, map-like area, which supports direct size comparison better than a rectangular cartogram with potentially misleading rectangle shapes and aspect ratios.

Evidence: In the controlled study, contiguous cartograms had the lowest error across compare, find top-k, and detect change tasks, and they were significantly more accurate than rectangular cartograms on all three tasks (Nusrat et al., 2018; Zeng & Battle, 2023).

• User Goal: Judge which region is bigger, which regions rank highest, or whether a region increased or decreased.
• Task: Compare areas, find top-k, or detect change in a cartogram.
• Data: Geospatial regions with values encoded by area.
• Chart Setting: A static cartogram where individual region areas must be compared directly.
• Success Criterion: Fewer size-comparison errors.

Break it when: The chart’s job is broad pattern summary rather than region-by-region area judgment. Why: Dorling cartograms performed best for summarize tasks and rectangular cartograms were not needed for that overview role.

Sacrifice: Perfect regularity of region shape. Risk: Some shapes still distort enough to make other tasks, such as exact shape recognition, harder. Mitigation: If the task changes to shape recognition, switch to a non-contiguous cartogram.

Mistake: Using a rectangular cartogram for region-size judgments. Why it fails: Rectangle forms and aspect ratios make region-by-region size judgments less accurate in practice.

Failure Sign: Readers miss which region is bigger, which ranks highest, or whether a region changed size. Quick Check: Ask one larger-than or top-k question on the current cartogram and on a contiguous alternative. Stronger Test: Compare error rates across a small set of compare, ranking, and change-detection questions.

• Replace the rectangular cartogram with a contiguous cartogram.
• Keep area as the value channel, but move the reader to connected, map-like regions rather than rectangles.
• If the chart only needs a broad overview, switch to a Dorling cartogram instead of keeping the rectangular form.