US Census Bureau shapefiles example

examples/README.md

@@ -54,6 +54,7 @@
 
 - [`loader-airtable`](https://observablehq.observablehq.cloud/framework-example-loader-airtable/) - Loading data from Airtable
 - [`loader-arrow`](https://observablehq.observablehq.cloud/framework-example-loader-arrow/) - Generating Apache Arrow IPC files
+- [`loader-census`](https://observablehq.observablehq.cloud/framework-example-loader-census/) - Loading and transform shapefiles from the U.S. Census Bureau
 - [`loader-databricks`](https://observablehq.observablehq.cloud/framework-example-loader-databricks/) - Loading data from Databricks
 - [`loader-duckdb`](https://observablehq.observablehq.cloud/framework-example-loader-duckdb/) - Processing data with DuckDB
 - [`loader-elasticsearch`](https://observablehq.observablehq.cloud/framework-example-loader-elasticsearch/) - Loading data from Elasticsearch

examples/loader-census/.gitignore

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+.DS_Store
+/dist/
+node_modules/
+yarn-error.log

examples/loader-census/README.md

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+[Framework examples →](../)
+
+# U.S. Census Bureau data loader
+
+View live: <https://observablehq.observablehq.cloud/framework-example-loader-census/>
+
+This Observable Framework example demonstrates how to write a bash data loader that downloads high-resolution shapefiles from the US Census Bureau, and converts them into a web-friendly format.

examples/loader-census/observablehq.config.js

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+export default {
+  root: "src"
+};

examples/loader-census/package.json

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+{
+  "type": "module",
+  "private": true,
+  "scripts": {
+    "clean": "rimraf src/.observablehq/cache",
+    "build": "rimraf dist && observable build",
+    "dev": "observable preview",
+    "deploy": "observable deploy",
+    "observable": "observable"
+  },
+  "dependencies": {
+    "@observablehq/framework": "latest",
+    "shapefile": "^0.6.6",
+    "topojson-client": "^3.1.0",
+    "topojson-server": "^3.0.1",
+    "topojson-simplify": "^3.0.3"
+  },
+  "devDependencies": {
+    "rimraf": "^5.0.5"
+  },
+  "engines": {
+    "node": ">=18"
+  }
+}

examples/loader-census/src/.gitignore

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+/.observablehq/cache/

examples/loader-census/src/data/ca.json.sh

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+# Download the ZIP archive from the Census Bureau (if needed).
+if [ ! -f src/.observablehq/cache/cb_2023_06_cousub_500k.zip ]; then
+  curl -o src/.observablehq/cache/cb_2023_06_cousub_500k.zip 'https://www2.census.gov/geo/tiger/GENZ2023/shp/cb_2023_06_cousub_500k.zip'
+fi
+
+# Unzip the ZIP archive to extract the shapefile.
+unzip -oqd src/.observablehq/cache src/.observablehq/cache/cb_2023_06_cousub_500k.zip
+
+# Convert the shapefile to TopoJSON, simplify, and merge counties.
+shp2json --encoding utf-8 -n src/.observablehq/cache/cb_2023_06_cousub_500k.shp > src/.observablehq/cache/cb_2023_06_cousub_500k.json
+
+geo2topo -q 1e5 -n counties=src/.observablehq/cache/cb_2023_06_cousub_500k.json \
+  | toposimplify -f -s 1e-7 \
+  | topomerge state=counties

examples/loader-census/src/index.md

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+# Census boundaries
+
+Here’s how to fetch high-resolution shapefiles from the [U.S. Census Bureau](https://www.census.gov/geographies/mapping-files/time-series/geo/cartographic-boundary.html) and convert them into a web-friendly format in a bash data loader.
+
+First, you’ll need to install a few packages:
+
+```sh
+npm install shapefile topojson-client topojson-server topojson-simplify
+```
+
+Next, here’s a bash script, `ca.json.sh`:
+
+```bash
+# Download the ZIP archive from the Census Bureau (if needed).
+if [ ! -f src/.observablehq/cache/cb_2023_06_cousub_500k.zip ]; then
+  curl -o src/.observablehq/cache/cb_2023_06_cousub_500k.zip 'https://www2.census.gov/geo/tiger/GENZ2023/shp/cb_2023_06_cousub_500k.zip'
+fi
+
+# Unzip the ZIP archive to extract the shapefile.
+unzip -oqd src/.observablehq/cache src/.observablehq/cache/cb_2023_06_cousub_500k.zip
+
+# Convert the shapefile to GeoJSON, then to TopoJSON, simplify, and merge counties.
+shp2json --encoding utf-8 -n src/.observablehq/cache/cb_2023_06_cousub_500k.shp > src/.observablehq/cache/cb_2023_06_cousub_500k.json
+geo2topo -q 1e5 -n counties=src/.observablehq/cache/cb_2023_06_cousub_500k.json \
+  | toposimplify -f -s 1e-7 \
+  | topomerge state=counties
+```
+
+The census.gov URL comes from the Census Bureau page linked above. Here “06” is the state of California’s FIPS code, and “cousub” means county subdivisions.
+
+And here’s the result displayed with Plot:
+
+```js echo
+Plot.plot({
+  width: 640,
+  height: 720,
+  projection: {
+    type: "conic-conformal",
+    parallels: [37 + 4 / 60, 38 + 26 / 60],
+    rotate: [120 + 30 / 60, 0],
+    domain: castate
+  },
+  marks: [
+    Plot.geo(castate),
+    Plot.geo(cacounties, {strokeOpacity: 0.2})
+  ]
+})
+```
+
+```js echo
+const ca = FileAttachment("data/ca.json").json();
+```
+
+```js echo
+const castate = topojson.feature(ca, ca.objects.state);
+const cacounties = topojson.mesh(ca, ca.objects.counties, (a, b) => a !== b);
+```