javascript - How to get all points inside a polygon in openlayers 4.x? - Stack Overflow

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I want to integrate a feature using Openlayers 4.x is that I want to get all points inside a polygon on map. Currently I can get all coordinates of polygon itself.

But I want all coordinates or points inside of the polygon. If I explain it more that means I want all points or coordinates on the map surrounded by the polygon area.

I want to integrate a feature using Openlayers 4.x is that I want to get all points inside a polygon on map. Currently I can get all coordinates of polygon itself.

But I want all coordinates or points inside of the polygon. If I explain it more that means I want all points or coordinates on the map surrounded by the polygon area.

• javascript
• geometry
• openlayers

Share Improve this question Follow edited Oct 30, 2020 at 13:15 mkrieger1 23.5k77 gold badges6464 silver badges8282 bronze badges asked Sep 25, 2018 at 13:08 Shahriat HossainShahriat Hossain 34044 silver badges1212 bronze badges

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3 Answers 3

Sorted by: Reset to default 3

Building on @willsters answer, if having identified the candidates and you are only looking for points (where the geometry is the extent) forEachFeatureIntersectingExtent could then be used in the reverse direction to see if the points intersect the polygon's geometry.

var candidates = [];
source.forEachFeatureIntersectingExtent(myPolygon.getGeometry().getExtent(),function(feature){
    if (feature.getGeometry().get('type') == 'Point') {
        candidates.push(feature);
    }
});

var selected = [];
candidates.forEach(function(candidate){
    source.forEachFeatureIntersectingExtent(candidate.getGeometry().getExtent(),function(feature){
        if (feature === myPolygon) {
            selected.push(candidate);
        }
    });
});

Also for single coordinate points I think it could be done in a single step:

var selected = [];
source.forEachFeatureIntersectingExtent(myPolygon.getGeometry().getExtent(),function(feature){
    if (feature.getGeometry().get('type') == 'Point' &&
        myPolygon.getGeometry().intersectsCoordinate(feature.getGeometry().get('coordinates')) {
            candidates.push(selected);
    }
});

Regarding dividing into cells something like this would generate pinpush points for each cell of a 10x10 grid which contained the polygon. If only part of a cell intersected the polygon a pinpush at the center of the cell might be outside the geometry.

var extent = myPolygon.getGeometry().getExtent();
for (var i=extent[0]; i<extent[2]; i+=(extent[2]-extent[0])/10) {
    for (var j=extent[1]; j<extent[3]; j+=(extent[3]-extent[1])/10) {
        var cellExtent = [i,j,i+(extent[2]-extent[0])/10),j+(extent[3]-extent[1])/10];
        source.forEachFeatureIntersectingExtent(cellExtent,function(feature){
            if (feature === myPolygon) {
               var pinPush = new ol.feature(new ol.geom.Point(ol.extent.getCenter(cellExtent)));
               source.addFeature(pinPush); 
            }
        });
    }
}

I often include the turf.js library alongside of openlayers specifically for tasks like this. Most of my geometries are nativley in geojson so turf.js is a perfect fit. If you have a geojson FeatureCollection. you can iterate the .features array (or even just any array of [x, y] points) and check each one to see if it's inside your polygon. I can make a working fiddle if it helps.

// In this example I'm looking for all features
// that have AT LEAST ONE point within  
// the world extent (WGS84)
const polygon = turf.bboxPolygon([-180, -90, 180, 90])
myGeoJson.features.forEach(function(feature){
    const points = turf.explode(feature);
    const pointsWithin = turf.pointsWithinPolygon(points, polygon);
    if(pointsWithin.features && pointsWithin.features.length){
        // feature has AT LEAST ONE point inside the polygon
        // I can see what points by iterating the
        // pointsWithin.features array
    }else{
        // feature has ZERO points inside the polgyon
    }
});

http://turfjs/docs#pointsWithinPolygon

Use vectorSource.forEachFeatureIntersectingExtent() with polygonGeom.getExtent(). That will get you the extent shortcut for most of them. After that you will need to either implement point-in-poly yourself (there are numerous sources online for this) or use a library like https://github./bjornharrtell/jsts. OpenLayers only provides geometry intersection with extents.

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