topic Re: find centroid of a polygon in Qt Maps SDK Questions

find centroid of a polygon

rsharma — Fri, 14 Aug 2020 10:34:15 GMT

i want to calculate centroid (center point ) of a polygon.

Re: find centroid of a polygon

LukeSmallwood — Fri, 14 Aug 2020 11:18:58 GMT

Hi rajni sharma‌ - you can use the GeometryEngine.labelPoint to give you a point that's within the polygon and near the center of gravity.

Re: find centroid of a polygon

rsharma — Fri, 14 Aug 2020 11:31:08 GMT

how to use it , is their any example i could follow, because i could not found any example for it.

Should i use it like this:

var a = Point labelPoint(Polygon ring);
it gives some syntax error

Re: find centroid of a polygon

LukeSmallwood — Fri, 14 Aug 2020 11:45:01 GMT

We have a number of samples that cover the use of GeometryEngine on github - for example look at this one.

For the label point call you should do something like:

var centroid = GeometryEngine.labelPoint(ring);

Re: find centroid of a polygon

rsharma — Mon, 17 Aug 2020 07:50:00 GMT

hi Luke Smallwood‌

I tried it your way, but when i try to console the qml point it crashed my application everytime.

Actually i am trying to put a picture marker on centre of polygon.

So i need to get centre x and y coordinates

Suppose this is the JSON m passing in ring

[[["172.4843305878348","-43.48191185124988"],["172.4877698503442","-43.48248171686965"],["172.48715361282302","-43.484065878559875"],["172.48428867831893","-43.48421950013839"],["172.4843305878348","-43.48191185124988"]]]

 function createPolygon(ring) {

        // create polygon using json

         const sr = { "wkid": 4326};

         var boundaryJson = {"rings":ring,"spatialReference":sr};

         var boundary= ArcGISRuntimeEnvironment.createObject("Polygon", {"json": boundaryJson,"id":"property"});

        var centroid = GeometryEngine.labelPoint(boundary);

        console.log("==========");

        console.log(JSON.stringify(centroid));//Crashed my application

        return boundary;

Re: find centroid of a polygon

LukeSmallwood — Mon, 17 Aug 2020 09:49:56 GMT

Hi rajni sharma‌ - I think this line is causing the problems because the returned Polygon will be null:

 var boundary= ArcGISRuntimeEnvironment.createObject("Polygon", {"json": boundaryJson,"id":"property"});

I think this should be:

 var boundary= ArcGISRuntimeEnvironment.createObject("Polygon", {json: boundaryJson});

(I'm not sure what the "id" : "property" is for here?)

Re: find centroid of a polygon

rsharma — Mon, 17 Aug 2020 09:54:26 GMT

my application still crashed. but when i do not stringify it, it return qml point sum hex number

 const sr = { "wkid": 4326};

         var boundaryJson = {"rings":ring,"spatialReference":sr};

         var boundary= ArcGISRuntimeEnvironment.createObject("Polygon", {"json": boundaryJson});

        var centroid = GeometryEngine.labelPoint(boundary);

        console.log("==========");

        console.log((centroid));

        return boundary;

Re: find centroid of a polygon

LukeSmallwood — Mon, 17 Aug 2020 10:18:57 GMT

Thanks rajni sharma‌ - I see what you are trying to do. This works for me:

        const ringJson = [[["172.4843305878348","-43.48191185124988"],["172.4877698503442","-43.48248171686965"],["172.48715361282302","-43.484065878559875"],["172.48428867831893","-43.48421950013839"],["172.4843305878348","-43.48191185124988"]]]

        var boundaryJson = {"rings":ringJson, "spatialReference":sr};

        var boundary= ArcGISRuntimeEnvironment.createObject("Polygon", {json: boundaryJson});

        console.log("boundary", boundary)

        console.log(JSON.stringify(boundary.json));

        var centroid = GeometryEngine.labelPoint(boundary);

        console.log("==========");

        console.log("centroid", centroid);

        console.log(JSON.stringify(centroid.json));

For reference, when you see "qml point sum hex number" you are printing the address of the Point object. To see the details of the point you need to either print property values or stringify the json:

console.log(JSON.stringify(centroid.json));

Re: find centroid of a polygon

rsharma — Mon, 17 Aug 2020 10:23:29 GMT

Thanks it worked. I was trying to stringify it, but didn't know, i have to stringify the json property of the point. Thanks a lot again