Sea Level (with WebGL)

Sea level + m
Go to San Francisco, New York, Mumbai, or Shanghai

Render sea level at different elevations

The style property of a WebGL tile layer accepts a color expression that can be used to modify pixel values before rendering. Here, RGB tiles representing elevation data are loaded and rendered so that values at or below sea level are blue, and values above sea level are transparent. The color expression operates on normalized pixel values ranging from 0 to 1. The band operator is used to select normalized values from a single band.

After converting the normalized RGB values to elevation, the case expression is used to pick colors to apply at a given elevation. Instead of using constant numeric values as the stops in the colors array, the var operator allows you to use a value that can be modified by your application. When the user drags the sea level slider, the layer.updateStyleVariables() function is called to update the level style variable with the value from the slider.

main.js

import ImageTile from 'ol/source/ImageTile.js';
import Map from 'ol/Map.js';
import TileLayer from 'ol/layer/WebGLTile.js';
import View from 'ol/View.js';
import {fromLonLat} from 'ol/proj.js';

const key = 'Get your own API key at https://www.maptiler.com/cloud/';
const attributions =
  '<a href="https://www.maptiler.com/copyright/" target="_blank">&copy; MapTiler</a> ' +
  '<a href="https://www.openstreetmap.org/copyright" target="_blank">&copy; OpenStreetMap contributors</a>';

// band math operates on normalized values from 0-1
// so we scale by 255 to align with the elevation formula
// from https://cloud.maptiler.com/tiles/terrain-rgb-v2/
const elevation = [
  '+',
  -10000,
  ['*', 0.1 * 255 * 256 * 256, ['band', 1]],
  ['*', 0.1 * 255 * 256, ['band', 2]],
  ['*', 0.1 * 255, ['band', 3]],
];

const layer = new TileLayer({
  opacity: 0.6,
  source: new ImageTile({
    url:
      'https://api.maptiler.com/tiles/terrain-rgb-v2/{z}/{x}/{y}.webp?key=' +
      key,
    tileSize: 512,
    maxZoom: 14,
  }),
  style: {
    variables: {
      level: 0,
    },
    color: [
      'case',
      // use the `level` style variable to determine the color
      ['<=', elevation, ['var', 'level']],
      [139, 212, 255, 1],
      [139, 212, 255, 0],
    ],
  },
});

const map = new Map({
  target: 'map',
  layers: [
    new TileLayer({
      source: new ImageTile({
        url:
          'https://api.maptiler.com/maps/streets-v2/{z}/{x}/{y}.png?key=' + key,
        attributions: attributions,
        tileSize: 512,
        maxZoom: 22,
      }),
    }),
    layer,
  ],
  view: new View({
    center: fromLonLat([-122.3267, 37.8377]),
    zoom: 11,
  }),
});

const control = document.getElementById('level');
const output = document.getElementById('output');
control.addEventListener('input', function () {
  output.innerText = control.value;
  layer.updateStyleVariables({level: parseFloat(control.value)});
});
output.innerText = control.value;

const locations = document.getElementsByClassName('location');
for (let i = 0, ii = locations.length; i < ii; ++i) {
  locations[i].addEventListener('click', relocate);
}

function relocate(event) {
  const data = event.target.dataset;
  const view = map.getView();
  view.setCenter(fromLonLat(data.center.split(',').map(Number)));
  view.setZoom(Number(data.zoom));
}

index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Sea Level (with WebGL)</title>
    <link rel="stylesheet" href="node_modules/ol/ol.css">
    <style>
      .map {
        width: 100%;
        height: 400px;
      }
      #level {
        display: inline-block;
        width: 150px;
        vertical-align: text-bottom;
      }

      a.location {
        cursor: pointer;
      }

      #map {
        background: #8bd4ff;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <label>
      Sea level
      <input id="level" type="range" min="0" max="100" value="1"/>
      +<span id="output"></span> m
    </label>
    <br>
    Go to
    <a class="location" data-center="-122.3267,37.8377" data-zoom="11">San Francisco</a>,
    <a class="location" data-center="-73.9338,40.6861" data-zoom="11">New York</a>,
    <a class="location" data-center="72.9481,18.9929" data-zoom="11">Mumbai</a>, or
    <a class="location" data-center="120.831,31.160" data-zoom="9">Shanghai</a>

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

package.json

{
  "name": "webgl-sea-level",
  "dependencies": {
    "ol": "10.2.1"
  },
  "devDependencies": {
    "vite": "^3.2.3"
  },
  "scripts": {
    "start": "vite",
    "build": "vite build"
  }
}