Tissot Indicatrix

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import Feature from 'ol/Feature.js';
import Map from 'ol/Map.js';
import View from 'ol/View.js';
import {circular as circularPolygon} from 'ol/geom/Polygon.js';
import {Tile as TileLayer, Vector as VectorLayer} from 'ol/layer.js';
import TileWMS from 'ol/source/TileWMS.js';
import VectorSource from 'ol/source/Vector.js';

var vectorLayer4326 = new VectorLayer({
  source: new VectorSource()
});

var vectorLayer3857 = new VectorLayer({
  source: new VectorSource()
});

var map4326 = new Map({
  layers: [
    new TileLayer({
      source: new TileWMS({
        url: 'https://ahocevar.com/geoserver/wms',
        params: {
          'LAYERS': 'ne:NE1_HR_LC_SR_W_DR',
          'TILED': true
        }
      })
    }),
    vectorLayer4326
  ],
  target: 'map4326',
  view: new View({
    projection: 'EPSG:4326',
    center: [0, 0],
    zoom: 2
  })
});

var map3857 = new Map({
  layers: [
    new TileLayer({
      source: new TileWMS({
        url: 'https://ahocevar.com/geoserver/wms',
        params: {
          'LAYERS': 'ne:NE1_HR_LC_SR_W_DR',
          'TILED': true
        }
      })
    }),
    vectorLayer3857
  ],
  target: 'map3857',
  view: new View({
    center: [0, 0],
    zoom: 2
  })
});

var radius = 800000;
var x, y;
for (x = -180; x < 180; x += 30) {
  for (y = -90; y < 90; y += 30) {
    var circle4326 = circularPolygon([x, y], radius, 64);
    var circle3857 = circle4326.clone().transform('EPSG:4326', 'EPSG:3857');
    vectorLayer4326.getSource().addFeature(new Feature(circle4326));
    vectorLayer3857.getSource().addFeature(new Feature(circle3857));
  }
}

<!DOCTYPE html>
<html>
  <head>
    <title>Tissot Indicatrix</title>
    <link rel="stylesheet" href="https://openlayers.org/en/v5.3.0/css/ol.css" type="text/css">
    <!-- The line below is only needed for old environments like Internet Explorer and Android 4.x -->
    <script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=requestAnimationFrame,Element.prototype.classList,URL"></script>

  </head>
  <body>
    <h4>EPSG:4326</h4>
    <div id="map4326" class="map"></div>
    <h4>EPSG:3857</h4>
    <div id="map3857" class="map"></div>
    <script>
      import Feature from 'ol/Feature.js';
      import Map from 'ol/Map.js';
      import View from 'ol/View.js';
      import {circular as circularPolygon} from 'ol/geom/Polygon.js';
      import {Tile as TileLayer, Vector as VectorLayer} from 'ol/layer.js';
      import TileWMS from 'ol/source/TileWMS.js';
      import VectorSource from 'ol/source/Vector.js';

      var vectorLayer4326 = new VectorLayer({
        source: new VectorSource()
      });

      var vectorLayer3857 = new VectorLayer({
        source: new VectorSource()
      });

      var map4326 = new Map({
        layers: [
          new TileLayer({
            source: new TileWMS({
              url: 'https://ahocevar.com/geoserver/wms',
              params: {
                'LAYERS': 'ne:NE1_HR_LC_SR_W_DR',
                'TILED': true
              }
            })
          }),
          vectorLayer4326
        ],
        target: 'map4326',
        view: new View({
          projection: 'EPSG:4326',
          center: [0, 0],
          zoom: 2
        })
      });

      var map3857 = new Map({
        layers: [
          new TileLayer({
            source: new TileWMS({
              url: 'https://ahocevar.com/geoserver/wms',
              params: {
                'LAYERS': 'ne:NE1_HR_LC_SR_W_DR',
                'TILED': true
              }
            })
          }),
          vectorLayer3857
        ],
        target: 'map3857',
        view: new View({
          center: [0, 0],
          zoom: 2
        })
      });

      var radius = 800000;
      var x, y;
      for (x = -180; x < 180; x += 30) {
        for (y = -90; y < 90; y += 30) {
          var circle4326 = circularPolygon([x, y], radius, 64);
          var circle3857 = circle4326.clone().transform('EPSG:4326', 'EPSG:3857');
          vectorLayer4326.getSource().addFeature(new Feature(circle4326));
          vectorLayer3857.getSource().addFeature(new Feature(circle3857));
        }
      }
    </script>
  </body>
</html>