I would like to show a polygon in OpenLayers. The polygon was defined in 3 KML files with difference level of detail, ie:

  1. Low Resolution KML (with 100 points)
  2. Medium Resolution KML (with 1000 points)
  3. High Resolution KML (with 10000 points)

First, I created 3 vector layers for each KML file with difference max and min resolution, ie:

  1. minRosolution: 0.0001716, maxResolution: 0.0004000
  2. minRosolution: 0.0000429, maxResolution: 0.0001716
  3. minRosolution: 0.0000000, maxResolution: 0.0000429

Then added those vector layers into openlayer list. It works but OpenLayers is not usable until the whole KML is being loaded. Hence, I would like to improve it by making the loading process for Medium and High resolution KML in the background. Is it possible?

1 Answer 1


If you specify a url in the source it isn't loaded until required for display. As long as the data is static you can load it by xhr/fetch and add it to the source immediately, such as in this alternative approach to one of the OpenLayers examples:

  var projection = ol.proj.get('EPSG:3857');

  var raster = new ol.layer.Tile({
    source: new ol.source.OSM()

  var vector = new ol.layer.Vector({
    source: new ol.source.Vector()

  var xhr = new XMLHttpRequest();
  xhr.onload = function() {
    vector.getSource().addFeatures(new ol.format.KML().readFeatures(xhr.responseText, {featureProjection: projection}));
  xhr.open("GET", 'https://openlayers.org/en/v4.6.5/examples/data/kml/2012-02-10.kml', true);

  var map = new ol.Map({
    layers: [raster, vector],
    target: document.getElementById('map'),
    view: new ol.View({
      center: [876970.8463461736, 5859807.853963373],
      projection: projection,
      zoom: 10
  • Hi Mike, thank you for the your reply. You are right, the hanging effect is not because of loading the KML but because of the downloading process from server, sorry about that. The file size of medium and high resolution KML is quite big, 1.5MB and 3MB respectively. I was thinking to use Web Worker to download the higher resolution KML. Do you have any suggestion ? Feb 26, 2019 at 7:15

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