Today I am going to give you a short introduction into the Google Earth Engine and show you how to create a cloud free mosaic of Europe using Sentinel-2 data in just one minute! Yes, literally one minute!

The Google Earth Engine is a “computing platform that allows users to run geospatial analysis on Google’s infrastructure.” This means that you don’t have to download any data yourself and that you can perform high speed data analysis of all kinds of available satellite imagery on your laptop, computer or even your cell phone if you wish by using the computantional power of Google.

1. Sign up at earthengine.google.com

No worries, it’s free of charge if you are a researcher or simply want to explore the possibilities and power of the Google Earth Engine. However if you want to use it for commercial purposes, you should get in touch with Goolge first.

2. Go to code.earthengine.google.com

Google Earth Engine offers a couple of interfaces. In our case study we are going to use the code editor interface. Via this interface you can make us of a variety of already existing code chunks which will help you with your analysis and the processing of all kinds of satellite imagery. You can find a short intro into the Google Earth Engine API here.

The following illustration shows the main features of the code editor:

Google Earth Engine Code Editor (Illustration by Google)

3. Select an existing script from the script manager

From the script manager on the left side select “Filtered Composite”. This script takes an polygon extent as an input:

var polygon = ee.Geometry.Polygon([[
 [-109.05, 37.0], [-102.05, 37.0], [-102.05, 41.0], // colorado
 [-109.05, 41.0], [-111.05, 41.0], [-111.05, 42.0], // utah
 [-114.05, 42.0], [-114.05, 37.0], [-109.05, 37.0]]]);

And then selects all available Landsat 7 scenens from April 2000 until July 2000:

var collection = ee.ImageCollection('LE7_L1T')
 .filterDate('2000-04-01', '2000-07-01')
 .filterBounds(polygon);

In a third step the median of all of these images is calculated and displayed to the map:

// Select the median pixel.
var median = collection.median();

// Select the red, green and blue bands.
var result = median.select('B3', 'B2', 'B1');
Map.addLayer(result, {gain: '1.4, 1.4, 1.1'});
Map.setCenter(-110, 40, 5);

The result is a Landsat 7 mosaic over Colorado and Denver. You can execute the code by clicking the “Run” button. It takes only a couple of seconds to compute! Amazing!

4. Pushing the limits – Europe Sentinel-2 cloud free. How long will it take?

Let’s test how long the same procedure will take for the entire continent of Europe. And lets use Sentinel-2 instead!
Any guess how long it will take? Here are the code adaptations I have made:

Polygon over Europe:

var geometry = /* color: #d63000 */ee.Geometry.Polygon( [[[-26.60888671875, 66.01801815922043], 
[-18.1494140625, 32.565333160841035], [15.954649288064843, 35.39010560236367], [44.80224609375, 36.756490329505176], 
[43.59375, 70.85188122123132], [11.88720703125, 73.23937702441908]]]);

var polygon = geometry;

This time we want Sentinel-2 data, so let’s change “LE7_L1T” to “COPERNICUS/S2” and specify the time frame to April 2016 until December 2016:

var collection = ee.ImageCollection('COPERNICUS/S2')
 .filterDate('2016-04-01', '2016-12-01')
 .filterBounds(polygon);

var min = collection.min();

// Select the red, green and blue bands.
var result = min.select('B4', 'B3', 'B2');

Map.addLayer(result, {gain: '0.1, 0.1, 0.1', scale:20});
Map.setCenter(5, 47, 4);

Voila! We are done! How long did it take? About a minute!! Wow, stunning! Here the result:

Cloud-free Sentinel-2 mosaic of Europe