avvertix / geojson-rewind

Enforce polygon ring winding order for GeoJSON in PHP

geojson-rewind

A set of PHP helper classes to assist in generating GeoJSON geometries that are compliant with the GeoJSON specification.

Polygon ring order was undefined in the original GeoJSON spec, but since RFC7946 the right hand rule is mandated.

A linear ring MUST follow the right-hand rule with respect to the area it bounds, i.e., exterior rings are counterclockwise, and holes are clockwise.

(If you know British English rather than American English, simply substitute anticlockwise for counterclockwise.)

Acknowledgements

geojson-rewind is a port to PHP of Mapbox's Node.JS geojson-rewind module by Tom McWright et al. Full credits, kudos and acknowledgements are due to Tom and the rest of the Mapbox team.

Install

The easiest way to install geojson-rewind is by using composer:

$> composer require vicchi/geojson-rewind

Usage

<?php

include "vendor/autoload.php";

$source = [
    'type' => 'Polygon',
    'coordinates' => [
    [ [100.0, 0.0], [101.0, 0.0], [101.0, 1.0], [100.0, 1.0], [100.0, 0.0] ],
    [ [100.2, 0.2], [100.8, 0.2], [100.8, 0.8], [100.2, 0.8], [100.2, 0.2] ]
    ]
];
$enforce_rfc7946=true;
$output = Vicchi\GeoJson\Rewind::rewind($source, $enforce_rfc7946);

// show output for demonstration purposes
var_dump($output);

?>

The output could look like this ($> is the command line prompt).

$> php test.php
array(2) {
  ["type"]=>
  string(7) "Polygon"
  ["coordinates"]=>
  array(2) {
    [0]=>
    array(5) {
      [0]=>
      array(2) {
        [0]=>
        float(100)
        [1]=>
        float(0)
      }
      [1]=>
      array(2) {
        [0]=>
        float(101)
        [1]=>
        float(0)
      }
      [2]=>
      array(2) {
        [0]=>
        float(101)
        [1]=>
        float(1)
      }
      [3]=>
      array(2) {
        [0]=>
        float(100)
        [1]=>
        float(1)
      }
      [4]=>
      array(2) {
        [0]=>
        float(100)
        [1]=>
        float(0)
      }
    }
    [1]=>
    array(5) {
      [0]=>
      array(2) {
        [0]=>
        float(100.2)
        [1]=>
        float(0.2)
      }
      [1]=>
      array(2) {
        [0]=>
        float(100.2)
        [1]=>
        float(0.8)
      }
      [2]=>
      array(2) {
        [0]=>
        float(100.8)
        [1]=>
        float(0.8)
      }
      [3]=>
      array(2) {
        [0]=>
        float(100.8)
        [1]=>
        float(0.2)
      }
      [4]=>
      array(2) {
        [0]=>
        float(100.2)
        [1]=>
        float(0.2)
      }
    }
  }
}

License

geojson-rewind is published under the BSD-3-Clause license. See License File for more information.