Edit

WMS GetLegendGraphic

Legend:

Example of a WMS GetLegendGraphic.

WMS supports the getLegendGraphic request. This example demonstrates the use of the getLegendUrl method.

As a legend can be responsive to the scale it is updated on every change of the resolution.

Copy 
index.html<!DOCTYPE html>
<html lang="en">
  <head>
    <title>WMS GetLegendGraphic</title>
    <!-- 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>
    <style>
      .map {
        width: 100%;
        height:400px;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    Legend:
    <div><img id="legend" src=""/></div>
    <script src="index.js"></script>
  </body>
</html>
Copy 
index.jsimport 'ol/ol.css';
import Map from 'ol/Map';
import View from 'ol/View';
import {Image as ImageLayer, Tile as TileLayer} from 'ol/layer';
import ImageWMS from 'ol/source/ImageWMS';
import OSM from 'ol/source/OSM';

var wmsSource = new ImageWMS({
  url: 'https://ahocevar.com/geoserver/wms',
  params: {'LAYERS': 'topp:states'},
  ratio: 1,
  serverType: 'geoserver'
});

var updateLegend = function(resolution) {
  var graphicUrl = wmsSource.getLegendUrl(resolution);
  var img = document.getElementById('legend');
  img.src = graphicUrl;
};

var layers = [
  new TileLayer({
    source: new OSM()
  }),
  new ImageLayer({
    extent: [-13884991, 2870341, -7455066, 6338219],
    source: wmsSource
  })
];

var map = new Map({
  layers: layers,
  target: 'map',
  view: new View({
    center: [-10997148, 4569099],
    zoom: 4
  })
});

// Initial legend
var resolution = map.getView().getResolution();
updateLegend(resolution);

// Update the legend when the resolution changes
map.getView().on('change:resolution', function(event) {
  var resolution = event.target.getResolution();
  updateLegend(resolution);
});
Copy 
package.json{
  "name": "wms-getlegendgraphic",
  "dependencies": {
    "ol": "6.1.1"
  },
  "devDependencies": {
    "parcel": "1.11.0"
  },
  "scripts": {
    "start": "parcel index.html",
    "build": "parcel build --experimental-scope-hoisting --public-url . index.html"
  }
}