Building an SVG Choropleth Map
A choropleth map shades geographic regions by data value, using SVG paths for boundaries and a color scale for encoding.
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Region Boundaries as SVG Paths
Each geographic region in a choropleth map is a path element whose d attribute traces that region's boundary, typically generated by projecting geographic coordinates from a GeoJSON or TopoJSON source into the SVG's flat coordinate space.
- Each region is a path element whose d attribute traces its boundary
- Boundaries come from projecting GeoJSON or TopoJSON coordinate data
- The projection converts geographic coordinates into flat SVG coordinates
Mapping Data Values to Region Fills
The data encoding works by joining each region's identifier to a data value, normalizing that value across the dataset's range, and applying an interpolated color from a sequential scale as the fill attribute on that region's path element.
- Join each region's identifier to its corresponding data value first
- Normalize the value across the dataset's full range for consistent scaling
- Apply the interpolated color as the fill attribute on that region's path
Frequently Asked Questions
Where do the region boundary paths come from?
They're typically generated by projecting GeoJSON or TopoJSON geographic data into SVG coordinates using a map projection function.
Should a choropleth use raw values or values normalized by population?
Normalizing by population or area is usually better, since raw counts largely reflect region size rather than the underlying phenomenon being mapped.
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