Earthquakes with custom symbols

KML 9 vector 63 style 17 canvas 6 symbol 2

Demonstrates the use of `toContext` to create custom icon symbols.

This example parses a KML file and renders the features as a vector layer. The layer is given a style that renders earthquake locations with a custom lightning symbol and a size relative to their magnitude.

main.js

import 'ol/ol.css';
import KML from 'ol/format/KML';
import Map from 'ol/Map';
import Polygon from 'ol/geom/Polygon';
import Stamen from 'ol/source/Stamen';
import VectorSource from 'ol/source/Vector';
import View from 'ol/View';
import {Fill, Icon, Stroke, Style} from 'ol/style';
import {Tile as TileLayer, Vector as VectorLayer} from 'ol/layer';
import {toContext} from 'ol/render';

const symbol = [
  [0, 0],
  [4, 2],
  [6, 0],
  [10, 5],
  [6, 3],
  [4, 5],
  [0, 0],
];
let scale;
const scaleFunction = function (coordinate) {
  return [coordinate[0] * scale, coordinate[1] * scale];
};

const styleCache = {};
const styleFunction = function (feature) {
  // 2012_Earthquakes_Mag5.kml stores the magnitude of each earthquake in a
  // standards-violating <magnitude> tag in each Placemark.  We extract it from
  // the Placemark's name instead.
  const name = feature.get('name');
  const magnitude = parseFloat(name.substr(2));
  const size = parseInt(10 + 40 * (magnitude - 5), 10);
  scale = size / 10;
  let style = styleCache[size];
  if (!style) {
    const canvas = document.createElement('canvas');
    const vectorContext = toContext(canvas.getContext('2d'), {
      size: [size, size],
      pixelRatio: 1,
    });
    vectorContext.setStyle(
      new Style({
        fill: new Fill({color: 'rgba(255, 153, 0, 0.4)'}),
        stroke: new Stroke({color: 'rgba(255, 204, 0, 0.2)', width: 2}),
      })
    );
    vectorContext.drawGeometry(new Polygon([symbol.map(scaleFunction)]));
    style = new Style({
      image: new Icon({
        img: canvas,
        imgSize: [size, size],
        rotation: 1.2,
      }),
    });
    styleCache[size] = style;
  }
  return style;
};

const vector = new VectorLayer({
  source: new VectorSource({
    url: 'data/kml/2012_Earthquakes_Mag5.kml',
    format: new KML({
      extractStyles: false,
    }),
  }),
  style: styleFunction,
});

const raster = new TileLayer({
  source: new Stamen({
    layer: 'toner',
  }),
});

const map = new Map({
  layers: [raster, vector],
  target: 'map',
  view: new View({
    center: [0, 0],
    zoom: 2,
  }),
});

index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Earthquakes with custom symbols</title>
    <!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer -->
    <script src="https://unpkg.com/elm-pep"></script>
    <!-- The line below is only needed for old environments like Internet Explorer and Android 4.x -->
    <script src="https://cdn.polyfill.io/v3/polyfill.min.js?features=fetch,requestAnimationFrame,Element.prototype.classList,URL,TextDecoder,Number.isInteger"></script>
    <style>
      .map {
        width: 100%;
        height:400px;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <script src="main.js"></script>
  </body>
</html>

package.json

{
  "name": "earthquake-custom-symbol",
  "dependencies": {
    "ol": "6.6.1"
  },
  "devDependencies": {
    "parcel": "^2.0.0-beta.1"
  },
  "scripts": {
    "start": "parcel index.html",
    "build": "parcel build --public-url . index.html"
  }
}