Circle To Polygon

The GeoJSON spec does not support circles. If you wish to create an area that represents a circle, your best bet is to create a polygon that roughly approximates the circle. In the limit of the number of edges becoming infinite, your Polygon will match a circle.

circleToPolygon([173.283966, -41.270634], 20000, { numberOfEdges: 32 }) would yield the polygon below:

There is also a port to Go/Golang that can be found here: https://github.com/chrusty/go-circle-to-polygon

Installation

npm install --save circle-to-polygon

or

yarn add circle-to-polygon

Usage

Example

const circleToPolygon = require("circle-to-polygon");

const coordinates = [173.283966, -41.270634]; //[lon, lat]
const radius = 200000; // in meters
const options = { numberOfEdges: 32 }; //optional, defaults to { numberOfEdges: 32 }

const polygon = circleToPolygon(coordinates, radius, options);

console.log(polygon);
/*
{
  type: "Polygon",
  coordinates: [
    [
      [173.283966, -39.47400343176097],
      [172.8297426608343, -39.50761945331798],
      [172.39166717580562, -39.607271255365916],
      [171.98544458449058, -39.76940340765346],
      [171.62589074038397, -39.98820144316868],
      [171.3264802848837, -40.255758887782214],
      [171.09888995216616, -40.56231121046952],
      [170.9525431282912, -40.89653624994988],
      [170.894168491739, -41.24591956982946],
      [170.92739416288478, -41.597181119390946],
      [171.0524081585746, -41.9367562214545],
      [171.26572430426506, -42.251319123422796],
      [171.56009750883513, -42.528331314494025],
      [171.9246304894919, -42.75659019219929],
      [172.3451031959859, -42.92674764018193],
      [172.80453558092947, -43.03176422124745],
      [173.283966, -43.06726456823905],
      [173.76339641907052, -43.03176422124745],
      [174.22282880401409, -42.92674764018193],
      [174.64330151050808, -42.75659019219929],
      [175.00783449116483, -42.528331314494025],
      [175.3022076957349, -42.251319123422796],
      [175.51552384142542, -41.9367562214545],
      [175.6405378371152, -41.597181119390946],
      [175.673763508261, -41.24591956982946],
      [175.61538887170875, -40.89653624994988],
      [175.46904204783382, -40.56231121046952],
      [175.24145171511628, -40.255758887782214],
      [174.94204125961602, -39.98820144316868],
      [174.58248741550943, -39.76940340765346],
      [174.17626482419436, -39.607271255365916],
      [173.73818933916564, -39.50761945331798],
      [173.283966, -39.47400343176097],
    ],
  ],
};
*/

Parameters

• coordinates Array of length 2 or 3 *required
  • First Element: longitude Number *required, can be any number <=180 and >=-180
  • Second Element: latitude Number *required, can be any number <=90 and >=-90
  • Third Element: Ignored if present
• radius Number *required, can be any number >0
• options Object or Number. Omitting this variable is same as passing { numberOfEdges: 32 } and passing a number is same as passing { numberOfEdges: <number> }
  • numberOfEdges Number can be any number >=3. Defaults to 32 when undefined
  • earthRadius Number can be any number >0. Defaults to 6378137 (equatorial Earth radius) when undefined
  • bearing Number can be any number. Defaults to 0 when undefined. How many degrees the circle should be rotated. (Most noticeable for "circles" with few edges.)
  • rightHandRule Boolean default to false when undefined. If true, the circle will be drawn in the opposite direction. This is useful when drawing a hole in another shape, or if your system is following the old standard.

Disclaimers

• Decimal values will not throw error for numberOfEdges! Instead one of the edges of the polygon will be smaller than the others. In other words, all edges will not have the same length if a decimal number is passed as numberOfEdges.
• A circle whose edge cross longitude edges (-180 or 180) or a latitude edge (-90 or 90) will contain coordinate points that are outside the standardized coordinates (eg: [182, 23]). This is because there are two ways to represent a line going from [179, x] to [181, y]. One way is simply writing it as [[179, x], [182, y]] while the other is to write it as a multi-polygon. There is a plan to support multi-polygons but it has not yet been implemented.

Authors

• Gabriel Zimmermann
• Johannes Jarbratt

Contributors

• Jan Žák

License

ISC