Function polygonCenter

libs/leaflet/leaflet-src.js:6223–6274

(latlngs, crs)

   * Returns the center ([centroid](http://en.wikipedia.org/wiki/Centroid)) of the passed LatLngs (first ring) from a polygon.
   */
  function polygonCenter(latlngs, crs) {
  	var i, j, p1, p2, f, area, x, y, center;

  	if (!latlngs || latlngs.length === 0) {
  		throw new Error('latlngs not passed');
  	}

  	if (!isFlat(latlngs)) {
  		console.warn('latlngs are not flat! Only the first ring will be used');
  		latlngs = latlngs[0];
  	}

  	var centroidLatLng = toLatLng([0, 0]);

  	var bounds = toLatLngBounds(latlngs);
  	var areaBounds = bounds.getNorthWest().distanceTo(bounds.getSouthWest()) * bounds.getNorthEast().distanceTo(bounds.getNorthWest());
  	// tests showed that below 1700 rounding errors are happening
  	if (areaBounds < 1700) {
  		// getting a inexact center, to move the latlngs near to [0, 0] to prevent rounding errors
  		centroidLatLng = centroid(latlngs);
  	}

  	var len = latlngs.length;
  	var points = [];
  	for (i = 0; i < len; i++) {
  		var latlng = toLatLng(latlngs[i]);
  		points.push(crs.project(toLatLng([latlng.lat - centroidLatLng.lat, latlng.lng - centroidLatLng.lng])));
  	}

  	area = x = y = 0;

  	// polygon centroid algorithm;
  	for (i = 0, j = len - 1; i < len; j = i++) {
  		p1 = points[i];
  		p2 = points[j];

  		f = p1.y * p2.x - p2.y * p1.x;
  		x += (p1.x + p2.x) * f;
  		y += (p1.y + p2.y) * f;
  		area += f * 3;
  	}

  	if (area === 0) {
  		// Polygon is so small that all points are on same pixel.
  		center = points[0];
  	} else {
  		center = [x / area, y / area];
  	}

  	var latlngCenter = crs.unproject(toPoint(center));
  	return toLatLng([latlngCenter.lat + centroidLatLng.lat, latlngCenter.lng + centroidLatLng.lng]);
  }

  /* @function centroid(latlngs: LatLng[]): LatLng
   * Returns the 'center of mass' of the passed LatLngs.