Using TopoJSON for SVG Maps
TopoJSON encodes shared boundaries once instead of duplicating them, producing much smaller files than equivalent GeoJSON.
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Why TopoJSON Files Are Smaller
In GeoJSON, a boundary shared between two adjacent regions is stored twice, once in each region's coordinate list, while TopoJSON stores that shared boundary as a single arc referenced by both regions, eliminating the duplication that dominates most map file sizes.
- GeoJSON stores a shared boundary twice, once per adjacent region
- TopoJSON stores it once as an arc that both regions reference
- Eliminating this duplication dramatically reduces overall file size
Converting TopoJSON Back to Renderable Features
Before rendering, TopoJSON must be converted back into standard feature geometry by resolving each region's arc references into a full coordinate list, after which the projection and path generation steps proceed exactly as they would with GeoJSON input.
- Arc references resolve back into full coordinate lists before rendering
- After conversion, the data behaves exactly like standard GeoJSON
- Projection and path generation proceed identically from that point
Frequently Asked Questions
How much smaller is TopoJSON compared to GeoJSON?
The savings depend on how many boundaries are shared, but for typical administrative boundary maps the reduction is often substantial since most borders are shared between two regions.
Does TopoJSON render differently than GeoJSON?
No, once arc references are resolved back into coordinates, the resulting paths are identical — TopoJSON is purely a storage and transmission optimization.
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