This example uses OpenLayers v10.9.0. The latest is v.
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#### WMS without Projection

wms 13 projection 14

#####

Example of two WMS layers using the projection EPSG:21781, which is unknown to the client.

As long as no coordinate transformations are required, OpenLayers works fine with projections that are only configured with a `code` and `units`.

##### main.js

    import Map from 'ol/Map.js';
    import View from 'ol/View.js';
    import ScaleLine from 'ol/control/ScaleLine.js';
    import {defaults as defaultControls} from 'ol/control/defaults.js';
    import ImageLayer from 'ol/layer/Image.js';
    import TileLayer from 'ol/layer/Tile.js';
    import Projection from 'ol/proj/Projection.js';
    import ImageWMS from 'ol/source/ImageWMS.js';
    import TileWMS from 'ol/source/TileWMS.js';

    const layers = [
      new TileLayer({
        source: new TileWMS({
          attributions:
            '© <a href="https://shop.swisstopo.admin.ch/en/products/maps/national/lk1000"' +
            'target="_blank">Pixelmap 1:1000000 / geo.admin.ch</a>',
          crossOrigin: 'anonymous',
          params: {
            'LAYERS': 'ch.swisstopo.pixelkarte-farbe-pk1000.noscale',
            'FORMAT': 'image/jpeg',
          },
          url: 'https://wms.geo.admin.ch/',
        }),
      }),
      new ImageLayer({
        source: new ImageWMS({
          attributions:
            '© <a href="https://www.hydrodaten.admin.ch/en/notes-on-the-flood-alert-maps.html"' +
            'target="_blank">Flood Alert / geo.admin.ch</a>',
          crossOrigin: 'anonymous',
          params: {'LAYERS': 'ch.bafu.hydroweb-warnkarte_national'},
          serverType: 'mapserver',
          url: 'https://wms.geo.admin.ch/',
        }),
      }),
    ];

    // A minimal projection object is configured with only the SRS code and the map
    // units. No client-side coordinate transforms are possible with such a
    // projection object. Requesting tiles only needs the code together with a
    // tile grid of Cartesian coordinates; it does not matter how those
    // coordinates relate to latitude or longitude.
    //
    // With no transforms available projection units must be assumed to represent
    // true distances. In the case of local projections this may be a sufficiently
    // close approximation for a meaningful (if not 100% accurate) ScaleLine control.

    const projection = new Projection({
      code: 'EPSG:21781',
      units: 'm',
    });

    const map = new Map({
      controls: defaultControls().extend([new ScaleLine()]),
      layers: layers,
      target: 'map',
      view: new View({
        center: [660000, 190000],
        projection: projection,
        zoom: 9,
      }),
    });

##### index.html

    <!DOCTYPE html>
    <html lang="en">
      <head>
        <meta charset="UTF-8">
        <title>WMS without Projection</title>
        <link rel="stylesheet" href="node_modules/ol/ol.css">
        <style>
          .map {
            width: 100%;
            height: 400px;
          }
        </style>
      </head>
      <body>
        <div id="map" class="map"></div>

        <script type="module" src="main.js"></script>
      </body>
    </html>

##### package.json

    {
      "name": "wms-no-proj",
      "dependencies": {
        "ol": "10.9.0"
      },
      "devDependencies": {
        "vite": "^3.2.3"
      },
      "scripts": {
        "start": "vite",
        "build": "vite build"
      }
    }