WMS GetLegendGraphic

GetLegendGraphic 1 getLegendUrl 1 WMS 12

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.

main.js

import ImageWMS from 'ol/source/ImageWMS.js';
import Map from 'ol/Map.js';
import OSM from 'ol/source/OSM.js';
import View from 'ol/View.js';
import {Image as ImageLayer, Tile as TileLayer} from 'ol/layer.js';

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

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

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

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

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

// Update the legend when the resolution changes
map.getView().on('change:resolution', function (event) {
  const resolution = event.target.getResolution();
  updateLegend(resolution);
});

index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>WMS GetLegendGraphic</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>
    Legend:
    <div><img id="legend"/></div>

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

package.json

{
  "name": "wms-getlegendgraphic",
  "dependencies": {
    "ol": "9.1.0"
  },
  "devDependencies": {
    "vite": "^3.2.3"
  },
  "scripts": {
    "start": "vite",
    "build": "vite build"
  }
}