County Circles

County Population Circles on US Map with D3.js!

This example displays COVID-19 case counts as scaled circles overlaid on a map of US counties. The visualization uses `d3.geoAlbersUsa()` for projection, `d3.geoPath` to render state boundaries from TopoJSON, and `d3.scaleSqrt` to map case counts to circle radii based on data from a CSV file. County centroids, computed with `geoPath.centroid`, position the circles, while a `d3.zoom` handler enables panning and zooming.

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County Topology

This graph shows the topology of U.S. counties as a network, where each node sits at a county centroid and links connect neighboring counties that share borders. The visualization uses simplified geometry from a TopoJSON file, rendering the nation and county boundaries as SVG paths via `d3.geoPath`. Node positions come from path centroids, while neighbor relationships derive from `topojson.neighbors` on the county geometries. Edges are drawn as straight lines between connected centroids, overlaid on the map to reveal adjacency patterns across the country.

U.S. Counties TopoJSON Mesh

This example renders a minimalist map of U.S. counties using a TopoJSON mesh from the U.S. Census Bureau’s shapefile, served via the `us-10m.v1.json` file. The SVG visualization displays county boundaries as thin black lines with rounded joins, and overlays small circles at each arc’s starting point, revealing the topological structure of the data. The code uses `d3.geoPath` to project the mesh and `topojson.mesh` to extract boundaries, while `topojson.transform` positions the circles. Rendering is entirely SVG-based with D3 v4, and the map uses no fill, focusing purely on geometric outlines.

County Bubbles

A map of the United States uses brown circles sized by county population, with a legend for reference and hover tooltips showing the county name and population. The visualization overlays these bubbles on a gray landmass with white state borders, placing each circle at its county’s centroid. Built with D3 v3, it loads a TopoJSON file containing merged county, state, and nation geometries, projects it via `d3.geo.albersUsa`, and draws the map with `d3.geo.path` while sizing bubbles with `d3.scale.sqrt`. Population data comes from the 2012 American Community Survey.

US Map with County Centroids using D3.js

A US map displays state boundaries in light green with small red circles marking county centroids, computed via `d3.geoPath` and `d3.geoCentroid` from TopoJSON data. The visualization uses D3 v5 and the `geoAlbersUsa` projection, rendering SVG shapes. County population and COVID-19 case data from a CSV file determines circle sizing, though the visible circles have a fixed radius. Zooming is enabled through `d3.zoom`, and the map is drawn from the us-atlas counties-10m.json dataset.

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Area Choropleth

This example shows a choropleth of U.S. counties where each county’s color encodes its geographic area on a logarithmic scale from brown (small) to steelblue (large). The map renders county boundaries as SVG paths using `d3.geo.path()` and TopoJSON, with state outlines drawn as white strokes. The fill color is computed by passing each path’s area, obtained via `path.area(d)`, into a `d3.scale.log()` with a domain of 10 to 500. The data comes from a local `us.json` TopoJSON file, and the visualization uses d3.v3.

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U.S. Counties TopoJSON

This map displays the boundaries of every county in the United States using data from a TopoJSON file fetched from `https://d3js.org/us-10m.v1.json`. It renders the county shapes as filled SVG paths, with a white mesh overlay showing county borders via `topojson.mesh`. Hovering over any county turns it red. The visualization is built with D3 v4, using `d3.geoPath` to project the geometry and `topojson.feature` to extract the features, relying on the `us-atlas` TopoJSON data source.

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