polygon-lookup

polygon-lookup

A data-structure for performing fast, accurate point-in-polygon intersections against (potentially very large) sets of polygons. PolygonLookup builds an R-tree, or bounding-box spatial index, for its polygons and uses it to quickly narrow down the set of candidate polygons for any given point. If there are any ambiguities, it'll perform point-in-polygon intersections to identify the one that really intersects. PolygonLookup operates entirely in memory, and works best for polygons with little overlap.

API

PolygonLookup(featureCollection)

• featureCollection (optional): A GeoJSON collection to optionally immediately load with .loadFeatureCollection().

PolygonLookup.search(x, y, limit)

Narrows down the candidate polygons by bounding-box, and then performs point-in-polygon intersections to identify the first n container polygon (with n = limit, even if more polygons really do intersect).

• x: the x-coordinate to search for
• y: the y-coordinate to search for
• limit optional: the upper bound for number of intersecting polygon found (default value is 1, -1 to return all intersecting polygons)
• return: the intersecting polygon if one was found; a GeoJson FeatureCollection if multiple polygons were found and limit > 1; otherwise, undefined.

PolygonLookup.loadFeatureCollection(featureCollection)

Stores a feature collection in this PolygonLookup, and builds a spatial index for it. The polygons and rtree can be accessed via the .polygons and .rtree properties.

• featureCollection (optional): A GeoJSON collection containing some Polygons/MultiPolygons. Note that MultiPolygons will get expanded into multiple polygons.

example usage

var PolygonLookup = require( 'polygon-lookup' );
var featureCollection = {
	type: 'FeatureCollection',
	features: [{
		type: 'Feature',
		properties: { id: 'bar' },
		geometry: {
			type: 'Polygon',
			coordinates: [ [ [ 0, 1 ], [ 2, 1 ], [ 3, 4 ], [ 1, 5 ] ] ]
		}
	}]
};
var lookup = new PolygonLookup( featureCollection );
var poly = lookup.search( 1, 2 );
console.log( poly.properties.id ); // bar