NDVI from a Sentinel 2 COG

NDVI:

Calculating NDVI and applying a custom color map.

The GeoTIFF layer in this example draws from two Sentinel 2 sources: a red band and a near-infrared band. The layer style includes a color expression that calculates the Normalized Difference Vegetation Index (NDVI) from values in the two bands. The interpolate expression is used to map NDVI values to colors. The layer.getData() method can be used to retrieve pixel values from the GeoTIFF. Move your mouse or tap on the map to see calculated NDVI values based on the red and near-infrared pixel values.

main.js

import Map from 'ol/Map.js';
import TileLayer from 'ol/layer/WebGLTile.js';
import GeoTIFF from 'ol/source/GeoTIFF.js';

const source = new GeoTIFF({
  sources: [
    {
      // visible red, band 1 in the style expression above
      url: 'https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/36/Q/WD/2020/7/S2A_36QWD_20200701_0_L2A/B04.tif',
      max: 10000,
    },
    {
      // near infrared, band 2 in the style expression above
      url: 'https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/36/Q/WD/2020/7/S2A_36QWD_20200701_0_L2A/B08.tif',
      max: 10000,
    },
  ],
});

const layer = new TileLayer({
  style: {
    color: [
      'interpolate',
      ['linear'],
      // calculate NDVI, bands come from the sources below
      ['/', ['-', ['band', 2], ['band', 1]], ['+', ['band', 2], ['band', 1]]],
      // color ramp for NDVI values, ranging from -1 to 1
      -0.2,
      [191, 191, 191],
      -0.1,
      [219, 219, 219],
      0,
      [255, 255, 224],
      0.025,
      [255, 250, 204],
      0.05,
      [237, 232, 181],
      0.075,
      [222, 217, 156],
      0.1,
      [204, 199, 130],
      0.125,
      [189, 184, 107],
      0.15,
      [176, 194, 97],
      0.175,
      [163, 204, 89],
      0.2,
      [145, 191, 82],
      0.25,
      [128, 179, 71],
      0.3,
      [112, 163, 64],
      0.35,
      [97, 150, 54],
      0.4,
      [79, 138, 46],
      0.45,
      [64, 125, 36],
      0.5,
      [48, 110, 28],
      0.55,
      [33, 97, 18],
      0.6,
      [15, 84, 10],
      0.65,
      [0, 69, 0],
    ],
  },
  source: source,
});

const map = new Map({
  target: 'map',
  layers: [layer],
  view: source.getView(),
});

const output = document.getElementById('output');
function displayPixelValue(event) {
  const data = layer.getData(event.pixel);
  if (!data) {
    return;
  }
  const red = data[0];
  const nir = data[1];
  const ndvi = (nir - red) / (nir + red);
  output.textContent = ndvi.toFixed(2);
}
map.on(['pointermove', 'click'], displayPixelValue);

index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>NDVI from a Sentinel 2 COG</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>
    <div>NDVI: <span id="output"></span></div>

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

package.json

{
  "name": "cog-math",
  "dependencies": {
    "ol": "10.4.0"
  },
  "devDependencies": {
    "vite": "^3.2.3"
  },
  "scripts": {
    "start": "vite",
    "build": "vite build"
  }
}