geojson-polygon-self-intersections

geojson-polygon-self-intersections

A very simple script to compute all self-intersections in a GeoJSON polygon.

According to the Simple Features standard, polygons may not self-intersect. GeoJSON, however, doesn't care about this. You can use this tool to check for self-intersections, list them or use them in some way.

This tool uses the rbush spatial index by default to speed up the detection of intersections. This is especially useful when are many edges but only few intersections. If you prefer, you can opt-out using an input parameter (see below) and it will perform a brute-force search for intersections instead. This might be preferable in case of few edges, as it allows you to avoid some overhead.

Usage

Get Node.js, then

npm install geojson-polygon-self-intersections

and use it like so:

var gpsi = require('geojson-polygon-self-intersections');

// poly = {type: "Feature", geometry: {type: "Polygon", coordinates: [[[1, 10], [11, 13], ...]]}}

var isects = gpsi(poly);

// isects = {type: "Feature", geometry: {type: "MultiPoint", coordinates: [[5, 8], [7, 3], ...]}}

Where poly is a GeoJSON Polygon, and isects is a GeoJSON MultiPoint.

Alternatively, you can use a filter function to specify the output. You have access to the following data per point:

• [x,y] intersection coordinates: isect
• ring index of the first edge: ring0
• edge index of the first edge: edge0
• [x,y] of the start point of the first edge: start0
• [x,y] of the end point of the first edge: end0
• fractional distance of the intersection on the first edge: frac0
• idem for the second edge: ring1, edge1, start1, end1, frac1
• boolean indicating if the intersection is unique: unique

Finally, you can set a boolean variable to specify if a spatial index should be used to filter for possible intersections.

Together, this may look like so:

var useSpatialIndex = 0;
var isects = gpsi(poly, function filterFn(isect, ring0, edge0, start0, end0, frac0, ring1, edge1, start1, end1, frac1, unique){return [isect, frac0, frac1];}, useSpatialIndex);

// isects = [[[5, 8], 0.4856, 0.1865]], [[[7, 3], 0.3985, 0.9658]], ...]