Convert geojson polygons into points with node using query-overpass and turf.js

Question

I use the node module "query-overpass" for a query to get farmshops from openstreetmaps. I would like to convert all polygons to points inside this script. I use turf.js to get the centroids of theese polygons, but I am not able to change the objects in a permanent way. This is my code so far:

const query_overpass = require("query-overpass");
const turf = require ("turf");
const fs = require("fs")
let test
let filename = "data/test.js"
let bbox = "48.91821286473131,8.309097290039062,49.0610446187357,8.520584106445312";
console.log('starting query for ' +filename)
console.log('bbox: ' +bbox)
let query = ` 
  [out:json][timeout:250];
  // gather results
  (
  // query part for: “vending=milk”
  node["vending"="milk"](${bbox});
  way["vending"="milk"](${bbox});
  relation["vending"="milk"](${bbox});
  // query part for: “shop=farm”
  node["shop"="farm"](${bbox});
  way["shop"="farm"](${bbox});
  relation["shop"="farm"](${bbox});
  // query part for: “vending=food”
  node["vending"="food"](${bbox});
  way["vending"="food"](${bbox});
  relation["vending"="food"](${bbox});
  );
  // print results
  out body;
  >;
  out skel qt;
`;

// query overpass, write result to file
query_overpass(query, (error, data)  => {
    data = JSON.stringify(data , null, 1)
    console.log(data)
    test = JSON.parse(data)


//create centroids for every polyon and save them as a point
    for (var i = 0; i < test.features.length; i++) { 
      console.log("Log: " +test.features[i].geometry.type)
      console.log("Log: " +test.features[i].properties.name)
      if (test.features[i].geometry.type === "Polygon"){
        console.log("polygon detected")
        var centroid = turf.centroid(test.features[i]);
            var lon = centroid.geometry.coordinates[0];
            var lat = centroid.geometry.coordinates[1];
            console.log(" lon: " +lon +" lat: " +lat)

            test.features[i].geometry.type = 'Point'
            //delete Polygon structure and insert centroids as new points here
            console.log("polygon deleted and changed to point")
      }

  }
  console.log(test)
   fs.writeFile(filename, `var file = ${test};` , ["utf-8"], (error, data) => {if (error) {console.log(error)}})
 }, {flatProperties: true}
)

It seems like I can change things inside of the for loop, but they do not appear when the data is saved later. It is basically a question of how to edit json objects properly, but I can't figure out why this doesnt work here at all.

So there are basically two questions:

• Why cant I override geometry.type in the example above?
• How can I delete the old polygon and add a new point to a feature?

Thanks for any help.

Answer 1

That's quite complicated... Why don't you let Overpass API do this job and use out center; instead of out body;>;out skel qt; to return the center points of all nodes, ways and relations. You can use overpass-turbo.eu to try this out first.