Finding Sentinel-2 imagery

Now that we’ve covered the principles, let’s start learning how to use satellite imagery! We’ll start with finding and downloading imagery from the internet. For this exercise, we’ll be using data from the Sentinel-2 mission.

Sentinel-2

The Sentinel-2 program is one of several remote sensing missions run by the European Space Agency (ESA). It consists of a pair of identical satellites orbiting the Earth at the same speed and direction, but at opposite points of the same orbit. This means that although each individual satellite revisits the same location at ten-day intervals, between them they can cover the whole Earth in a maximum of five days. The revisit time is shorter the further you get from the equator, down to 2-3 days for mid-latitudes.

The imaging equipment on the Sentinel-2 satellites includes sensors in the red, green and blue part of the visible light spectrum, with a resolution of 10m. The first satellite was launched in 2015, and the second in 2017 – this means that greater temporal resolution is available from 2017. The imagery is publicly available on the ESA’s Copernicus Open Access Hub, including the latest data which is constantly updated.

Copernicus Open Access Hub

The Copernicus Open Access Hub is where you can easily access the ESA Sentinel-2 imagery, and all their other freely available remote sensing data. To download from the site, you will need to create an account and then log in using the User Button at the top-right of the main page.

_{Logging in to the Copernicus Open Access Hub.}

Now we can search for some imagery!

Selecting an area of interest

• 1) Use the mouse to navigate to your area of interest. You can use the scroll wheel to zoom in and out, and left click, hold and drag to pan around the map.
• 2) To select your area, use a series of right-clicks to draw a polygon, and a double right-click to finish. We recommend choosing a small area for now, such as your hometown, or somewhere you know well.

You can also right click and drag the mouse to select a rectangular region instead, if you prefer.

_{Selecting an area of interest.}

• 3) Click the “Advanced Search” button to the left of the search bar and the basket icon.
• 4) Tick “Mission: Sentinel-2” and then click on “Insert search criteria…” and click the “Search” button.

_{Advanced Search tab.}

Filtering your results

You will probably find that you have hundreds or thousands of results depending on the size of your search area. As well as making your search area smaller (which helps a lot!) we will show you how to narrow down your results so you have a more manageable set.

• 1) Click the “Filter” button (where the “Advanced Search” button was).
• 2) Change the two dates in the “Sensing period” section at the top – try the last month or so.
• 3) Click into the search bar again and click the search button.

For a small area of interest, doing this should reduce your results from over a thousand to well under a hundred.

_{Viewing search results after filtering.}

We can also improve the quality of our results, as well as reducing the number, by removing cloudy images.

• 4) Click the “Filter” button and scroll down to “Cloud Cover %” in the Sentinel-2 section.
• 5) Type “[0 TO 10]” in the box, click the search bar and then click the search button.

This will remove the cloudier images from your results. If you get no results, you may have to increase “Cloud Cover %” from 10% to 25%, or 50% depending on the weather in your area of interest!

Browsing your results

Once you have a manageable number of results (less than 25 is a good starting point), you can then browse your imagery to find which image you would like to download.

• 1) Scroll through your results to see a RBG preview of the images in your area of interest.

If you want more information to help you chose, the “Sensing Date” (when the image was taken) is also provided for each result.

• 2) Find an image that covers your area best and has few or no clouds, and then click the “View Product Details” button (it looks like an eye!).

_{Browsing our results and the “View Product Details” button.}

Here you can find even more information about your particular image – have a browse and see what data is available. “Cloud cover percentage” is a good field to look for! If you want to choose a different image you can browse through the results by clicking the “Left” or “Right” buttons, or you can close the window and open the details of a different image.

_{The Image Details window.}

_{What area of interest are you searching for? Are there enough low cloud scenes for you to pick from?}