Single Image WMS with Proj4js

Example of integrating Proj4js for coordinate transforms.

With Proj4js integration, OpenLayers can transform coordinates between arbitrary projections.

<!DOCTYPE html>
<html>
  <head>
    <title>Single Image WMS with Proj4js</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>
    <div id="map" class="map"></div>
    <script>
      import Map from 'ol/Map.js';
      import View from 'ol/View.js';
      import {defaults as defaultControls, ScaleLine} from 'ol/control.js';
      import ImageLayer from 'ol/layer/Image.js';
      import {fromLonLat} from 'ol/proj.js';
      import Projection from 'ol/proj/Projection.js';
      import ImageWMS from 'ol/source/ImageWMS.js';
      import {register} from 'ol/proj/proj4.js';
      import proj4 from 'proj4';


      // Transparent Proj4js support:
      //
      // EPSG:21781 is known to Proj4js because its definition is registered by
      // calling proj4.defs(). Now when we create an ol/proj/Projection instance with
      // the 'EPSG:21781' code, OpenLayers will pick up the transform functions from
      // Proj4js. To get the registered ol/proj/Projection instance with other
      // parameters like units and axis orientation applied from Proj4js, use
      // `ol/proj#get('EPSG:21781')`.
      //
      // Note that we are setting the projection's extent here, which is used to
      // determine the view resolution for zoom level 0. Recommended values for a
      // projection's validity extent can be found at https://epsg.io/.

      proj4.defs('EPSG:21781',
        '+proj=somerc +lat_0=46.95240555555556 +lon_0=7.439583333333333 +k_0=1 ' +
          '+x_0=600000 +y_0=200000 +ellps=bessel ' +
          '+towgs84=660.077,13.551,369.344,2.484,1.783,2.939,5.66 +units=m +no_defs');
      register(proj4);

      var projection = new Projection({
        code: 'EPSG:21781',
        extent: [485869.5728, 76443.1884, 837076.5648, 299941.7864]
      });

      var extent = [420000, 30000, 900000, 350000];
      var layers = [
        new ImageLayer({
          extent: extent,
          source: new ImageWMS({
            url: 'https://wms.geo.admin.ch/',
            crossOrigin: 'anonymous',
            attributions: '© <a href="http://www.geo.admin.ch/internet/geoportal/' +
                'en/home.html">Pixelmap 1:1000000 / geo.admin.ch</a>',
            params: {
              'LAYERS': 'ch.swisstopo.pixelkarte-farbe-pk1000.noscale',
              'FORMAT': 'image/jpeg'
            },
            serverType: 'mapserver'
          })
        }),
        new ImageLayer({
          extent: extent,
          source: new ImageWMS({
            url: 'https://wms.geo.admin.ch/',
            crossOrigin: 'anonymous',
            attributions: '© <a href="http://www.geo.admin.ch/internet/geoportal/' +
                'en/home.html">National parks / geo.admin.ch</a>',
            params: {'LAYERS': 'ch.bafu.schutzgebiete-paerke_nationaler_bedeutung'},
            serverType: 'mapserver'
          })
        })
      ];

      var map = new Map({
        controls: defaultControls().extend([
          new ScaleLine()
        ]),
        layers: layers,
        target: 'map',
        view: new View({
          projection: projection,
          center: fromLonLat([8.23, 46.86], projection),
          extent: extent,
          zoom: 2
        })
      });
    </script>
  </body>
</html>