Published
Edited
Mar 8, 2021
chart = memChart(year)
dcCensusBlockGroupsUrl = FileAttachment('dc-census.ndjson').url()
getNdjsonStream = async url => {
const { body } = await fetch(url);
const reader = ndjsonStream(body).getReader();
return reader;
}
borders = FileAttachment('dc-borders-topo.json').json()
getProjection = ({ rotate, center, parallels, transverseMercator } = {}) => {
let _projection = transverseMercator
? d3.geoTransverseMercator()
: d3.geoConicConformal().parallels(parallels);
return _projection.rotate(rotate).center(center);
}
projection = {
const { width, height } = dimensions;
const marylandStatePlane = {
rotate: [77, 0],
center: [0, 37.66],
parallels: [38.3, 39.45]
};
return getProjection(marylandStatePlane).fitSize(
[width, height],
topojson.feature(borders, borders.objects['dc-borders'])
);
}
generateDensity = (feature, ethnicity_column, year, context) => {
if (feature === undefined || feature === null) return;
const { properties } = feature;
const bounds = geopath.bounds(feature);
const populationData = Math.round(
properties[`${ethnicity_column}${censusYearMap[year]}`] / peoplePerPoint
);
if (!populationData) return;
// https://github.com/d3/d3-geo#path_bounds
const x_min = bounds[0][0];
const x_max = bounds[1][0];
const y_min = bounds[0][1];
const y_max = bounds[1][1];
let hits = 0;
let count = 0;
const limit = populationData * 10; // limit test to 10x the population.
let points = [];
while (hits < populationData - 1 && count < limit) {
const lat = y_min + Math.random() * (y_max - y_min);
const lng = x_min + Math.random() * (x_max - x_min);
const randomPoint = turf.point([lng, lat], {
color: colorScheme[ethnicity_column]
});
if (turf.booleanPointInPolygon(randomPoint, feature)) {
points.push(randomPoint);
hits++;
}
count++;
}
return turf.featureCollection(points);
}
drawDensity = ({ features }, context) => {
for (const feature of features) {
const {
properties: { color }
} = feature;
const {
geometry: {
coordinates: [x, y]
}
} = feature;
context.fillStyle = color;
context.fillRect(x, y, 1, 1);
}
return context;
}
drawBorder = (topojsonCollection, context) => {
geopath.context(context)(
d3.geoProject(topojson.mesh(topojsonCollection), projection)
);
context.globalAlpha = 1;
context.strokeStyle = strokeColor;
context.stroke();
return context;
}
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