polygongeojsonoffsetcomputational-geometryjts

Inconsistent buffer distance using JSTS

My end goal is to get an inner band N meters wide for a given polygon.

I wanted to use JSTS's buffer, then substract the inside buffered polygon to the starting polygon to get the band.

The problem is that I get inconsistent width between the buffered polygon and the original one: Image of incosistent buffer

import GeoJSONWriter from "jsts/org/locationtech/jts/io/GeoJSONWriter";
import GeoJSONReader from "jsts/org/locationtech/jts/io/GeoJSONReader";
import { BufferOp } from "jsts/org/locationtech/jts/operation/buffer";

function getOffsetPolygonJSTS(
  poly: Feature<Polygon>,
  offsetSize: number
): Feature<Polygon> {
  const geoReader = new GeoJSONReader();
  const geoWriter = new GeoJSONWriter();
  const geometry = geoReader.read(poly.geometry);
  const res = BufferOp.bufferOp(geometry, -0.0001);
  const jstsBuffer = geoWriter.write(res);
  return polygon(jstsBuffer.coordinates);
}
Solution

  • It sounds like you're operating on (lat,long) coordinates and expecting the results to work like (x,y) coordinates. They do not, except near the equator.

    Perform the operation in a locally anisotropic coordinate space. That'll also be essential for specifying the width in meters, rather than guessing.