Use a topology-preserving cartogram for adjacency lookup

Choose a topology-preserving cartogram when readers must identify neighboring regions. For example, use a contiguous or rectangular cartogram instead of a non-contiguous or Dorling cartogram for find-the-neighbor tasks.

Neighbor lookup depends on shared borders, and readers make many more mistakes when those borders are broken or only approximated.

Mechanism: A topology-preserving cartogram keeps adjacency explicit, so readers can answer neighbor questions from the cartogram itself instead of inferring them from approximate proximity.

Evidence: In the controlled study, contiguous and rectangular cartograms substantially outperformed non-contiguous and Dorling cartograms on the find adjacency task, with significant error advantages for topology-preserving designs over the non-topology-preserving ones (Nusrat et al., 2018; Zeng & Battle, 2023).

• User Goal: Determine which regions share a border.
• Task: Find adjacency in a cartogram.
• Data: Geospatial regions where topological relations matter.
• Chart Setting: A static cartogram is being used for neighbor lookup rather than only for broad overview.
• Success Criterion: Lower adjacency error.

• Break it when: The main task is recognizing a region by its original shape. Why: Non-contiguous cartograms preserve shape better for that task.
• Break it when: The main task is only broad pattern summary. Why: Dorling cartograms were better for summarize tasks and do not need exact adjacency.

Sacrifice: Some shape or position fidelity, depending on the cartogram type. Risk: A rectangular cartogram can become too schematic for other map-reading tasks. Mitigation: If the chart also needs stronger geographic recognizability, use a contiguous cartogram rather than a rectangular one.

Mistake: Expecting a non-contiguous or Dorling cartogram to support neighbor finding because nearby regions still look close. Why it fails: Proximity is not a reliable substitute for preserved shared borders.

Failure Sign: Readers confuse touching regions with merely nearby regions. Quick Check: Ask one neighbor-finding question on the current cartogram and on a topology-preserving alternative. Stronger Test: Compare adjacency error on the same set of questions across the two cartogram types.

• Replace the current cartogram with a contiguous cartogram or a rectangular cartogram.
• Preserve shared borders between regions instead of relying on proximity alone.
• If the current rectangular cartogram is too schematic for the rest of the task, switch to a contiguous cartogram.