Inside
Whether a point is inside a polygon
function isPointInPolygon(point, polygon) {
const x = point[0], y = point[1];
let inside = false;
for (let i = 0, j = polygon.length - 1; i < polygon.length; j = i++) {
const xi = polygon[i][0], yi = polygon[i][1];
const xj = polygon[j][0], yj = polygon[j][1];
const intersect = ((yi > y) !== (yj > y)) &&
(x < (xj - xi) * (y - yi) / (yj - yi) + xi);
if (intersect) inside = !inside;
}
return inside;
}
// Example usage:
const point = [51.5074, -0.1278]; // Point to check (latitude, longitude)
const polygon = [
[51.508, -0.128], // Point 1
[51.509, -0.129], // Point 2
[51.509, -0.127], // Point 3
[51.508, -0.126] // Point 4
];
console.log(isPointInPolygon(point, polygon)); // Outputs true or false
Centroid
Simple
// Define the latitude and longitude points
const coordinates = [
{ lat: 37.7749, lon: -122.4194 }, // San Francisco
{ lat: 34.0522, lon: -118.2437 }, // Los Angeles
{ lat: 36.1699, lon: -115.1398 }, // Las Vegas
{ lat: 40.7128, lon: -74.0060 }, // New York
{ lat: 25.7617, lon: -80.1918 }, // Miami
{ lat: 41.8781, lon: -87.6298 }, // Chicago
{ lat: 47.6062, lon: -122.3321 } // Seattle
];
// Calculate the centroid of the coordinates
function calculateCentroid(coords) {
if (coords.length === 0) return { lat: 0, lon: 0 };
let sumLat = 0;
let sumLon = 0;
coords.forEach(coord => {
sumLat += coord.lat;
sumLon += coord.lon;
});
const avgLat = sumLat / coords.length;
const avgLon = sumLon / coords.length;
return { lat: avgLat, lon: avgLon };
}
const centroid = calculateCentroid(coordinates);
Complex
A way of creating a "centre" point that lies WITHIN funny shapes.
- How can you find the centroid of a concave irregular polygon in JavaScript? - Stack Overflow
- polylabel.js ยท GitHub
(function(f){if(typeof exports==="object"&&typeof module!=="undefined"){module.exports=f()}else if(typeof define==="function"&&define.amd){define([],f)}else{var g;if(typeof window!=="undefined"){g=window}else if(typeof global!=="undefined"){g=global}else if(typeof self!=="undefined"){g=self}else{g=this}g.polylabel = f()}})(function(){var define,module,exports;return (function e(t,n,r){function s(o,u){if(!n[o]){if(!t[o]){var a=typeof require=="function"&&require;if(!u&&a)return a(o,!0);if(i)return i(o,!0);var f=new Error("Cannot find module '"+o+"'");throw f.code="MODULE_NOT_FOUND",f}var l=n[o]={exports:{}};t[o][0].call(l.exports,function(e){var n=t[o][1][e];return s(n?n:e)},l,l.exports,e,t,n,r)}return n[o].exports}var i=typeof require=="function"&&require;for(var o=0;o<r.length;o++)s(r[o]);return s})({1:[function(require,module,exports){
'use strict';
var Queue = require('tinyqueue');
module.exports = polylabel;
function polylabel(polygon, precision, debug) {
precision = precision || 1.0;
// find the bounding box of the outer ring
var minX, minY, maxX, maxY;
for (var i = 0; i < polygon[0].length; i++) {
var p = polygon[0][i];
if (!i || p[0] < minX) minX = p[0];
if (!i || p[1] < minY) minY = p[1];
if (!i || p[0] > maxX) maxX = p[0];
if (!i || p[1] > maxY) maxY = p[1];
}
var width = maxX - minX;
var height = maxY - minY;
var cellSize = Math.min(width, height);
var h = cellSize / 2;
// a priority queue of cells in order of their "potential" (max distance to polygon)
var cellQueue = new Queue(null, compareMax);
// cover polygon with initial cells
for (var x = minX; x < maxX; x += cellSize) {
for (var y = minY; y < maxY; y += cellSize) {
cellQueue.push(new Cell(x + h, y + h, h, polygon));
}
}
// take centroid as the first best guess
var bestCell = getCentroidCell(polygon);
var numProbes = cellQueue.length;
while (cellQueue.length) {
// pick the most promising cell from the queue
var cell = cellQueue.pop();
// update the best cell if we found a better one
if (cell.d > bestCell.d) {
bestCell = cell;
if (debug) console.log('found best %d after %d probes', Math.round(1e4 * cell.d) / 1e4, numProbes);
}
// do not drill down further if there's no chance of a better solution
if (cell.max - bestCell.d <= precision) continue;
// split the cell into four cells
h = cell.h / 2;
cellQueue.push(new Cell(cell.x - h, cell.y - h, h, polygon));
cellQueue.push(new Cell(cell.x + h, cell.y - h, h, polygon));
cellQueue.push(new Cell(cell.x - h, cell.y + h, h, polygon));
cellQueue.push(new Cell(cell.x + h, cell.y + h, h, polygon));
numProbes += 4;
}
if (debug) {
console.log('num probes: ' + numProbes);
console.log('best distance: ' + bestCell.d);
}
return [bestCell.x, bestCell.y];
}
function compareMax(a, b) {
return b.max - a.max;
}
function Cell(x, y, h, polygon) {
this.x = x; // cell center x
this.y = y; // cell center y
this.h = h; // half the cell size
this.d = pointToPolygonDist(x, y, polygon); // distance from cell center to polygon
this.max = this.d + this.h * Math.SQRT2; // max distance to polygon within a cell
}
// signed distance from point to polygon outline (negative if point is outside)
function pointToPolygonDist(x, y, polygon) {
var inside = false;
var minDistSq = Infinity;
for (var k = 0; k < polygon.length; k++) {
var ring = polygon[k];
for (var i = 0, len = ring.length, j = len - 1; i < len; j = i++) {
var a = ring[i];
var b = ring[j];
if ((a[1] > y !== b[1] > y) &&
(x < (b[0] - a[0]) * (y - a[1]) / (b[1] - a[1]) + a[0])) inside = !inside;
minDistSq = Math.min(minDistSq, getSegDistSq(x, y, a, b));
}
}
return (inside ? 1 : -1) * Math.sqrt(minDistSq);
}
// get polygon centroid
function getCentroidCell(polygon) {
var area = 0;
var x = 0;
var y = 0;
var points = polygon[0];
for (var i = 0, len = points.length, j = len - 1; i < len; j = i++) {
var a = points[i];
var b = points[j];
var f = a[0] * b[1] - b[0] * a[1];
x += (a[0] + b[0]) * f;
y += (a[1] + b[1]) * f;
area += f * 3;
}
return new Cell(x / area, y / area, 0, polygon);
}
// get squared distance from a point to a segment
function getSegDistSq(px, py, a, b) {
var x = a[0];
var y = a[1];
var dx = b[0] - x;
var dy = b[1] - y;
if (dx !== 0 || dy !== 0) {
var t = ((px - x) * dx + (py - y) * dy) / (dx * dx + dy * dy);
if (t > 1) {
x = b[0];
y = b[1];
} else if (t > 0) {
x += dx * t;
y += dy * t;
}
}
dx = px - x;
dy = py - y;
return dx * dx + dy * dy;
}
},{"tinyqueue":2}],2:[function(require,module,exports){
'use strict';
module.exports = TinyQueue;
function TinyQueue(data, compare) {
if (!(this instanceof TinyQueue)) return new TinyQueue(data, compare);
this.data = data || [];
this.length = this.data.length;
this.compare = compare || defaultCompare;
if (data) for (var i = Math.floor(this.length / 2); i >= 0; i--) this._down(i);
}
function defaultCompare(a, b) {
return a < b ? -1 : a > b ? 1 : 0;
}
TinyQueue.prototype = {
push: function (item) {
this.data.push(item);
this.length++;
this._up(this.length - 1);
},
pop: function () {
var top = this.data[0];
this.data[0] = this.data[this.length - 1];
this.length--;
this.data.pop();
this._down(0);
return top;
},
peek: function () {
return this.data[0];
},
_up: function (pos) {
var data = this.data,
compare = this.compare;
while (pos > 0) {
var parent = Math.floor((pos - 1) / 2);
if (compare(data[pos], data[parent]) < 0) {
swap(data, parent, pos);
pos = parent;
} else break;
}
},
_down: function (pos) {
var data = this.data,
compare = this.compare,
len = this.length;
while (true) {
var left = 2 * pos + 1,
right = left + 1,
min = pos;
if (left < len && compare(data[left], data[min]) < 0) min = left;
if (right < len && compare(data[right], data[min]) < 0) min = right;
if (min === pos) return;
swap(data, min, pos);
pos = min;
}
}
};
function swap(data, i, j) {
var tmp = data[i];
data[i] = data[j];
data[j] = tmp;
}
},{}]},{},[1])(1)
});