As a browser library: geojson-rewind.js

https://bundle.run/geojson-rewind

Hi there! I've got a set of polygons that I cannot rewind with geojson-rewind.

GeoJSON: https://gist.github.com/stepankuzmin/f12965a84eee3eed6df89e65d48696ae

Steps to reproduce:

const fs = require('fs');
const geojsonhint = require('@mapbox/geojsonhint');
const rewind = require('geojson-rewind');

const geojson = JSON.parse(fs.readFileSync('./invalid.geojson'));
const errors = geojsonhint.hint(geojson);
console.log(errors[0].message); // Polygons and MultiPolygons should follow the right-hand rule

const rewindedGeojson = rewind(geojson);
const newErrors = geojsonhint.hint(rewindedGeojson);
console.log(newErrors[0].message); // Polygons and MultiPolygons should follow the right-hand rule

geojson-rewind 0.3.1 and github:mapbox/geojson-rewind#v0.4.0
@mapbox/geojsonhint 2.1.0

There is a critical prototype pollution security issue in minimist 1.2.5 so that the package version has to be bumped to 1.2.6
see https://www.npmjs.com/package/minimist/v/1.2.6#security

For anyone, using this library within TypeScript (and together with d3), you can use

declare module "@mapbox/geojson-rewind" {
    import {ExtendedFeature, ExtendedFeatureCollection, ExtendedGeometryCollection, GeoGeometryObjects} from "d3-geo"

    export default function rewind(
        geojson: ExtendedFeature | ExtendedFeatureCollection | GeoGeometryObjects | ExtendedGeometryCollection,
        clockwise: boolean,
    ): ExtendedFeature | ExtendedFeatureCollection | GeoGeometryObjects | ExtendedGeometryCollection;
}

If people are interested, I guess I could submit a PR with this code in an index.d.ts.

In the rewindRing function, the following line exists:

err += Math.abs(area) >= Math.abs(k) ? area - m + k : k - m + area;

I'm not familiar enough with the domain-specific calculations, but from a purely mathematical standpoint, the following computations are equivalent, regardless of the relative absolute values of area and k:

area - m + k
k - m + area

I don't know if one of those expressions is incorrect, and thus should be fixed, or if the comparison of absolute values can simply be eliminated and either computation can always be used.

They just get ignored.

@mapbox/geojson-area is the only dependency actually used at runtime by the package.

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-7598

Bundle.run has been discontinued, and for this reason the browser module suggested in the README doesn't work anymore.

Please introduce some bundler like browserify or rollup, or even better, convert this into an es module, since now node supports those as well.

I am strangely finding certain Esri products of late outputting GeoJSON as UTF with a byte order mark. I noticed your rewind code crashes on these files.

Cheers,
Paul

bom_example.json.txt

rewind currently doesn't handle a feature that looks like

    {
      "type": "Feature",
      "geometry": {
        "type": "Polygon",
        "coordinates": []
      }
    }

I'm not super familiar with GeoJSON (maybe empty coordinates is invalid?), but it worked for me on http://geojson.io

in case this isn't intentional, this seems like an easy enough fix

here: https://github.com/mapbox/geojson-rewind/blob/master/geojson-rewind#L11

It looks like the current implementation only exports a rewind function that returns (and sometimes mutates) a geojson object.

I need a function that will check for the "Right Hand Rule" - ensure polygon geoms have counter-clockwise external rings, clockwise internal rings. I don't have any need for the altered geometry, just the boolean.

The best I've got so far:

var rewind = require("geojson-rewind");
function isRightHandRule(geom) {
    // Hack to deepcopy geom and avoid mutation
    geomstring = JSON.stringify(geom);
    geomcopy = JSON.parse(geomstring);

    // Does the original geometry match the clockwise-wound geometry
    return geomstring === JSON.stringify(rewind(geomcopy, true));
}

But this has some major flaws: it needs to deepcopy the geom via JSON strings, it relies on string comparison and it needs to rewind the entire geometry rather than failing fast on the first linear ring that doesn't match.

Any ideas on a better way to implement this?

Was moved to @mapbox/geojson-area - should be a quick update