Use non-contiguous cartograms for region shape recognition
For lookup tasks on geospatial cartograms, prefer a shape-preserving cartogram type on distorted map views to improve fidelity and mitigate region-shape identification errors for readers matching regions back to the original map.
- purpose:refine
- basis:empirical
- task:retrieve
- chart:map
- data:geospatial
- quality:fidelity:use
- lever:chart-family
- reading-mode:lookup
advice
Choose a shape-preserving cartogram type
Use a non-contiguous cartogram when readers must recognize a region by its original outline. For example, replace a contiguous cartogram with a non-contiguous cartogram for map-to-cartogram shape-matching tasks.
reason
Why shape preservation works here
Preserving the original region outline removes the extra step of decoding a deformed polygon before the reader can match it to the source geography.
Mechanism: A non-contiguous cartogram keeps each region’s original shape, so readers can match outlines directly instead of inferring them from a distorted boundary.
Evidence: In the controlled study, non-contiguous cartograms produced significantly lower error than contiguous cartograms on the recognize task, while completion time was not significantly different (Nusrat et al., 2018; Zeng & Battle, 2023).
context
Use when shape matching is the main job
- User Goal: Match a region in the cartogram back to its original geographic shape.
- Task: Identify which distorted region corresponds to a region from the source map.
- Data: Geospatial regions where outline recognition matters.
- Chart Setting: A static cartogram is being used as a map-like view rather than only as an abstract summary.
- Success Criterion: Fewer recognition errors.
exceptions
Do not use when shared borders matter more than outlines
Break it when: The main task is finding which regions touch each other. Why: Non-contiguous cartograms do not preserve adjacency, and they performed poorly on neighbor-finding.
costs
Tradeoffs of shape preservation
Sacrifice: Shared borders between regions. Risk: The map can become sparse and visually fragmented. Mitigation: If adjacency must remain readable, switch to a contiguous cartogram instead of forcing the task onto a non-contiguous one.
mistakes
Common failure mode for shape recognition
Mistake: Asking readers to recognize region shapes in a cartogram that replaces regions with circles, rectangles, or heavily deformed polygons. Why it fails: The original outlines are no longer directly available for matching.
check
Check whether shape recognition is being supported
Failure Sign: Readers confuse which cartogram region corresponds to the original map region. Quick Check: Ask a reviewer to match one highlighted source-map region to the correct cartogram region. Stronger Test: Compare recognition error on the current cartogram type against a non-contiguous version using the same matching questions.
fix
Fix the recognition failure
- Replace the current cartogram type with a non-contiguous cartogram.
- Keep regions in their geographic positions while preserving their original outlines.
- If the chart also needs neighbor lookup, switch to a contiguous cartogram rather than keeping a non-contiguous one.