Sea Level

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var key = 'Your Mapbox access token from http://mapbox.com/ here';
var elevation = new ol.source.XYZ({
  url: 'https://api.mapbox.com/v4/mapbox.terrain-rgb/{z}/{x}/{y}.pngraw?access_token=' + key,
  crossOrigin: 'anonymous',
  transition: 0
});

var raster = new ol.source.Raster({
  sources: [elevation],
  operation: flood
});

var map = new ol.Map({
  target: 'map',
  layers: [
    new ol.layer.Tile({
      source: new ol.source.XYZ({
        url: 'https://api.mapbox.com/styles/v1/tschaub/ciutc102t00c62js5fqd47kqw/tiles/256/{z}/{x}/{y}?access_token=' + key
      })
    }),
    new ol.layer.Image({
      opacity: 0.6,
      source: raster
    })
  ],
  view: new ol.View({
    center: ol.proj.fromLonLat([-122.3267, 37.8377]),
    zoom: 11
  })
});

var control = document.getElementById('level');
var output = document.getElementById('output');
control.addEventListener('input', function() {
  output.innerText = control.value;
  raster.changed();
});
output.innerText = control.value;

raster.on('beforeoperations', function(event) {
  event.data.level = control.value;
});

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

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

<!DOCTYPE html>
<html>
  <head>
    <title>Sea Level</title>
    <link rel="stylesheet" href="https://openlayers.org/en/v4.6.4/css/ol.css" type="text/css">
    <!-- The line below is only needed for old environments like Internet Explorer and Android 4.x -->
    <script src=""></script>
    <script src="https://openlayers.org/en/v4.6.4/build/ol.js"></script>
    <style>
      #level {
        display: inline-block;
        width: 150px;
        vertical-align: text-bottom;
      }

      a.location {
        cursor: pointer;
      }
    </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>
      function flood(pixels, data) {
        var pixel = pixels[0];
        if (pixel[3]) {
          var height = -10000 + ((pixel[0] * 256 * 256 + pixel[1] * 256 + pixel[2]) * 0.1);
          if (height <= data.level) {
            pixel[0] = 145;
            pixel[1] = 175;
            pixel[2] = 186;
            pixel[3] = 255;
          } else {
            pixel[3] = 0;
          }
        }
        return pixel;
      }

      var key = 'Your Mapbox access token from http://mapbox.com/ here';
      var elevation = new ol.source.XYZ({
        url: 'https://api.mapbox.com/v4/mapbox.terrain-rgb/{z}/{x}/{y}.pngraw?access_token=' + key,
        crossOrigin: 'anonymous',
        transition: 0
      });

      var raster = new ol.source.Raster({
        sources: [elevation],
        operation: flood
      });

      var map = new ol.Map({
        target: 'map',
        layers: [
          new ol.layer.Tile({
            source: new ol.source.XYZ({
              url: 'https://api.mapbox.com/styles/v1/tschaub/ciutc102t00c62js5fqd47kqw/tiles/256/{z}/{x}/{y}?access_token=' + key
            })
          }),
          new ol.layer.Image({
            opacity: 0.6,
            source: raster
          })
        ],
        view: new ol.View({
          center: ol.proj.fromLonLat([-122.3267, 37.8377]),
          zoom: 11
        })
      });

      var control = document.getElementById('level');
      var output = document.getElementById('output');
      control.addEventListener('input', function() {
        output.innerText = control.value;
        raster.changed();
      });
      output.innerText = control.value;

      raster.on('beforeoperations', function(event) {
        event.data.level = control.value;
      });

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

      function relocate(event) {
        var data = event.target.dataset;
        var view = map.getView();
        view.setCenter(ol.proj.fromLonLat(data.center.split(',').map(Number)));
        view.setZoom(Number(data.zoom));
      }
    </script>
  </body>
</html>