Here's the problem: I have geoJSON and topoJSON files that give me the polygons for Census block groups and voting precincts. I'm trying to see by how much a given Census block group overlaps with a given precinct.
I've seen a couple examples of what I'm after in other languages—i.e. R and in some GIS tools—but I'm trying to write this as a Node.js script. A few questions:
In the end, the final product would look something like this—imagining that I have arrays of precincts and blockgroups, and each one is an object with a geometry property that contains the polygon data for the precinct or block group, and also imagining that I have a function called overlap
that, when passed two polygons spits out the percent overlap:
// Iterate over each precinct.
_.each( precincts, function ( precinct ) {
// Iterate over each blockgroup.
_.each( blockgroups, function ( blockgroup ) {
// Get the overlap for the current precinct and blockgroup.
var o = overlap( precinct.geometry, blockgroup.geometry );
// If they overlap at all...
if ( o > 0 ) {
// ...Add information about the overlap to the precinct.
precinct.overlaps.push({
blockgroup: blockgroup.id,
overlap: o
});
}
}
}
(I've seen this module, but that only gives if the polygons overlap, not by how much they do.)
turf-intersect takes two polygons and returns a polygon representing the intersection.
geojson-area takes a polygon and returns the area in square meters.
npm install turf
npm install geojson-area
var turf = require('turf');
var geojsonArea = require('geojson-area');
var poly1 = {
"type": "Feature",
"geometry": {
"type": "Polygon",
"coordinates": [[
[-122.801742, 45.48565],
[-122.801742, 45.60491],
[-122.584762, 45.60491],
[-122.584762, 45.48565],
[-122.801742, 45.48565]
]]
}
}
var poly2 = {
"type": "Feature",
"geometry": {
"type": "Polygon",
"coordinates": [[
[-122.520217, 45.535693],
[-122.64038, 45.553967],
[-122.720031, 45.526554],
[-122.669906, 45.507309],
[-122.723464, 45.446643],
[-122.532577, 45.408574],
[-122.487258, 45.477466],
[-122.520217, 45.535693]
]]
}
}
var intersection = turf.intersect(poly1, poly2);
var area_intersection = geojsonArea.geometry(intersection.geometry);
var area_poly1 = geojsonArea.geometry(poly1.geometry);
var percent_poly1_covered_by_poly2 = (area_intersection / area_poly1)*100;
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