Find central ploygon

Balasaheb Molawade 136

Hi,

We are using the Bing Maps API to plot polygons on a map. We plot around 10K to 15K polygons, and we want to find the central polygon to show the name of the plotted region. Is there any way to find the central polygon and display a label on it?

This is necessary because we plot multiple regions on the map, each containing thousands of polygons. To easily understand which region is in which area, displaying a label is the best option.

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rbrundritt 18,686 Reputation points Microsoft Employee

2024-06-20T16:01:56.9666667+00:00
There are a lot of different ways to do this as "central" is a bit ambiguous. Here are a couple examples of what "central polygon" could mean:

Take the bounding box (or bounding circle) of all polygons you have and find the polygon that is in the center of that. It's possible that no polygon is under the center coordinate.

Merge all the polygons together to create one big polygon, then find it's centroid and use that as there center. Again its possible that no polygon is under the center coordinate.

There is a geospatial center and a visual center, these are not the same as the map projection stretches the more you move towards the poles. If your map displays all the data nicely, it is likely centered based on the visual center of the bounding box.

Center could also be calculated based on the center of mass of coordinates, or based on distances between edges (e.g. Voronoi diagram).

For 1 and 2, a good example of when a center may not intersect a polygon is if the shape of the overall area is like a C. In your case, if no polygon intersects the "center" point, you could simply grab the nearest polygon.

Bing Maps does have a built-in spatial math library that could be used, however, given the recent retirement announcement of Bing Maps, relying on an open source like [Turf.js] would be smart and make it easy to migrate to Azure Maps in the future (Azure Maps is uses open data standards which work with tons of open-source libraries).

I've put together a code sample showing several different methods here: https://rbrundritt.azurewebsites.net/Demos/AzureMaps/PolygonCollectionCenter/index.html
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Balasaheb Molawade 136 Reputation points

2024-06-21T12:17:36.7333333+00:00

Hi Ricky,

Our team worked on it but is having a hard time getting it to work. Do you have sample code for Bing Maps?

We do not need the exact center of the polygon, but approximately near the center.

Thanks!

rbrundritt 18,686 Reputation points Microsoft Employee

2024-06-21T17:53:16.2566667+00:00

If not already, I'd start by converting your data to GeoJSON using the GeoJSON module, or ahead of time. This will make a lot of open source libraries easy to use and make a migration to Azure Maps in the future easier.

The solutions I provided in the sample here https://rbrundritt.azurewebsites.net/Demos/AzureMaps/PolygonCollectionCenter/index.html are based on GeoJSON, and simply put the results on Azure Maps. So reusing the main solution should be as easy as converting the your data to GeoJSON, then converting the resulting GeoJSON back to Bing Maps shapes. Start off by loading the required libraries if you are using one of the Turf or polylabel solutions. You can download a browser friendly version of polylabel here: Download polylabel for the browser here: https://github.com/mapbox/polylabel/files/6552548/polylabel.zip

Here is a Bing Maps version of the code sample: https://rbrundritt.azurewebsites.net/Demos/BingMaps/PolygonCollectionCenter/

Balasaheb Molawade 136 Reputation points

2024-06-24T12:52:17.95+00:00

Hi Ricky,

Thank you for your response, and sorry to bother you again.

As we read data using the API and plot it on the map, the polygon collection in the format shown below, which we cannot convert into GeoJSON.

Is there a way to find the center from the polygon collection.

Thanks

rbrundritt 18,686 Reputation points Microsoft Employee

2024-06-24T14:16:18.2333333+00:00

Since you are starting with Bing Maps shapes, you can still convert them to GeoJSON. The GeoJSON write method is meant to convert a single shape. So you can loop through your data to create a feature collection. If you want to be able to get the original polygon after all the calculations, you can capture the ID of the polygon when converting that to GeoJSON, then use that later to look up the polygon you want. Here is a code sample:

var shapes = layer.getPrimitives(); Microsoft.Maps.loadModule('Microsoft.Maps.GeoJson', function () { //Create a feature collection. var featureCollection = { type: 'FeatureCollection', features: [] }; //Loop through shapes for(var i = 0; i < shapes.length; i++){ //Create a GeoJSON feature from the geometry. featureCollection.features.push({ type: 'Feature', //Convert the shape to a GeoJSON geometry. geometry: Microsoft.Maps.GeoJson.write(shapes[i]), //Capture the id of the shape. id: shapes[i].id, properties: {} }); } //TODO: Add your logic to get central polygon... var centerPolygon = featureCollection.features[0]; //Just grabbing the first one for testing. //Look up the original shape. var centralShape; for(var i = 0; i < shapes.length; i++){ if(shapes[i].id === centerPolygon.id){ centralShape = shapes[i]; break; } } //Do something with the original shape. });

Balasaheb Molawade 136 Reputation points

2024-06-26T09:50:06.5333333+00:00

Hi Ricky,

Thanks for your response. We will review it.

Thanks!
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