OpenLayers – How to Get the Bigger Polygon of a Multipolygon

geometrymulti-polygonopenlayerspolygon

I have a Multipolygon that has 5 separate polygons and I want to get only the geometry of the biggest polygon.

How can I get the largest polygon from the multipolygon?

Best Answer

If you sort the polygons in descending size order it will be the first entry in the array

multipolygon.getPolygons().sort(function(polygon1, polygon2) {return polygon2.getArea() - polygon1.getArea();})[0]

Related Solutions

[GIS] OpenLayers 4.2 – Changing the interaction geometry type from polygon to multipolygon

The reason for that is because dragBox.getGeometry() returns bbox geometry. You should get geometry from features that intersect with bbox. Lets suppose that geojson file that is loaded have both polygon and multipolygon geometry type. The function in this case for 'bboxend' event is as follow.

  var selectedMultiPolygon;
dragBox.on('boxend', function() {
    selectedMultiPolygon = new ol.geom.MultiPolygon();
    var extent = dragBox.getGeometry().getExtent();
    source.forEachFeatureIntersectingExtent(extent,
        function(feature) {
            //check geometry type
            if (feature.getGeometry().getType() == 'MultiPolygon') {
                var polygonsArray = feature.getGeometry().getPolygons();
                for (i = 0; i < polygonsArray.length; i++) {
                    selectedMultiPolygon.appendPolygon(polygonsArray[i]);
                }
            } else if (feature.getGeometry().getType() == 'Polygon') {

            selectedMultiPolygon.appendPolygon(feature.getGeometry());
        }

        selectedFeatures.push(feature);

    });
console.log(selectedMultiPolygon);
});

Here is the fiddle: https://jsfiddle.net/v3yqf78L/

GeoJSON – Largest Polygon Inside a Multipolygon Using Leaflet

max_area_polygon;
max_area = 0 ;

for(poly in (feature.geometry.coordinates)){                              
    polygon = turf.polygon((feature.geometry.coordinates)[poly])
    area = turf.area(polygon); 

    if(area > max_area){
        max_area = area
        max_area_polygon = polygon // polygon with the largest area
    }
}
center = turf.centerOfMass(max_area_polygon);

Center point rather than centroid because for some polygons may have centroids out of the boundaries

With this calculated center you can bind a tooltip as it returns a GeoJSON point

Best Answer

Related Solutions

[GIS] OpenLayers 4.2 – Changing the interaction geometry type from polygon to multipolygon

GeoJSON – Largest Polygon Inside a Multipolygon Using Leaflet

Related Question