Downloading, Visualizing and Preprocessing Sentinel-2 Data in SNAP

Getting Started

Why do we need to take these steps

explanation of why it is being done to the data and what is being done

Getting Sentinel-2 Data

There are three ways we will outline in this tutorial to access Sentinel-2 data, through two Copernicus data servers, Scihub and SNAP Product Library, and Google Earth Explorer.

Copernicus account

1. Navigate to the Copernicus open access hub through this link 2. click the person icon with a question mark located in the upper right hand corner of your screen. 3. In the lower left corner of the drop down, click Sign Up and fill out all the necessary fields of the corresponding forms, being sure to enter a valid email address. 4. once your account has been created and validated, sign in using the same dropdown in step 3.

We now have access to our Copernicus and therefore Sentinel data. For more help, this link provides a comprehensive overview of the steps necessary https://scihub.copernicus.eu/userguide/SelfRegistration

Scihub

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Figure 1: Scihub Display page

In Figure 1 we see the default Scihub display. depending on the data we are looking for, we are able to filter our search by location, sensor, and in Sentinel-2's case, even satellite.

Below the login icon there are three Icons (figure 2)

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Figure 2: Scihub Navigation Icons

the first or top one show the map layers. Click through the options and note the Sentinel-2 Cloudless + Overlay. the second and third icons are in a slider format. the upper icon allows for navigation around the globe. Use this to find your area of interest. Once found switch the slider to the lower icon. This setting allows you to select an area of interest (AOI) which is your first filter for Sentinel data.

Next, navigate to the three horizontal lines icon, found on upper left corner of figure 1. this opens the advanced search dropdown, which allows for the filtering of available data by metrics of sensing and ingestion period, mission and the associated characteristics of each. After filtering, click the search icon to search the repository for images that meet both your included location and advanced search criteria.

If any images match your criteria the advance search dropdown will be replaced with the results.

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Subsetting to AOI

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It is beneficial to subset the downloaded Sentinel data package if your AOI is only a portion of the downloaded image. This will increase efficiency, reduce processing time as well as increase clarity of intent.

The simplest method is to navigate to Raster toolbar then the Subset option. In the popup window (FIGURE) a blue bounding box will appear, this represents the extent of your image or view. Move the edges of the box to your desired location to resize, then press OK. If your desired AOI has exact coordinates or measurements, this option is not the most ideal.

A few other ways to accomplish a subset are discussed in the Subset section of the [1] Basic Processing of Radarsat-1 data in Snap ESA tutorial found on the main Wiki page.

Exploring Data in SNAP

Navigation Tab

This tab, found in the default view at the lower left corner of the SNAP screen (figure 1) is a very useful tool for data exploration and comparison. This section will provide an overview of its capabilities. If the navigation tab is not immediately apparent, the navigation tab can be opened through this path: View -> Tool Windows -> Navigation in the right panel of the navigation tab, there are four magnifying glasses and two other special characters

1. The first is the Zoom in icon, which allows for investigation and visualization at a larger scale or higher resolution.
2. Next, Zoom out, which opposes the Zoom In tool.
3. The magnifying glass with the 'P' shows the the MSI at it highest granular pixel size. In other words the furthest extent one can zoom into the image, to a pixel level.
4. The magnifying glass containing an 'A' opposingly yet similarly pans to the furthest extent of the MSI. In other words, by clicking this icon, the image zooms out to show the entire image or scene.

The next two icons in the line up are extremely useful when comparing two expressions of the same scene. Open two of the image composites explored earlier (like the real colour and and infrared composite). They should be visualized as tabs in your main or major area that you can click between. Then use the path Window -> Tile Vertically to create a split screen of sorts with both images.

1. The first special character synchronizes views across multiple windows, so the exact same frame at the exact same resolution is being shown when panning in one image or in the navigation bar.
2. The second synchronizes the cursor across multiplie views, so the cursor point in the master scene can be identified easily in the drone/slave scenes.

Both these actions can be used concurrently. This configuration can be used to highlight finer resolution details and help drive analyses.

Note: There are also options to tile horizontally, evenly and singly. Choose the option which best displays the active image.

RGB Exploration

RGB image profiles

Sentinel-2 Multispectral data has 13 bands which can span from the 400nm to 2100nm on the electromagnetic scale. Any visual representation of a multispectral image however can only display three bands at any time, using the Red, Green and Blue colour signatures. It is thus possible to change and manipulate the bands used to understand more about the landscape or focus of the image.

To do this, open an RGB Image Window either by right clicking the file name and scrolling to Open RGB Image Window or by following the path Window -> Open RGB Image Window. A popup will appear which outlines the current orientation of bands to colours. A true colour image can be illustrated by matching red: band 4, green: band 3 and blue: band 2. click Ok and wait as the RGB image loads. To save this colour composite as a band, check the box next to Store RGB channels as virtual bands in current product. This option is useful when you need to toggle quickly between two composites. Play around with the possible colour combinations and think about what the arrangements are telling you.

Resampling to increase Usefulness

All bands in the Sentinel-2 product are not of equal resolution, so you may have had trouble combining some bands in your exploration (FIG). This is a problem as it restricts analyses and visualization techniques. To combat this, resampling of the bands is conducted using the following steps:

1. Navigate to the resampling tool using the path, Raster -> Geometric -> Resampling.
2. in the I/O parameters tab, Select the product on which the resampling will be conducted. There is also an option to rename the output file, by default is keeps the input name and appends resampled.
3. Select the Resampling Parameters tab. In the section Define size of resampled product, There are three option by which you can determine the resolution of your new product's bands,
   • by reference band of source product
   • by target width and height
   • by pixel resolution

Using the first option, select a visible wavelength band as the reference band, like band 2 or band 3.

1. Accept all default parameters for the section Define resampling algorithm.
2. A popup will appear notifying of the completed process and its delayed implementation. Click Okay.

1. Ensure Resample on pyramid levels (for faster imaging) is checked then Run.

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References

Geospatial Ecology and Remote Sensing. (2018a, March 13). Exploring Sentinel-2 multi-spectral band combinations in SNAP. https://www.youtube.com/watch?v=vtlN5MXYGaY

Geospatial Ecology and Remote Sensing. (2018b, March 13). Exploring spectral response curves from Sentinel-2 in SNAP. https://www.youtube.com/watch?v=E8Tu_IaJl6c&list=PLf6lu3bePWHCOUjTDZRNx5N07otDM8iUq&index=2

SNAP - Explore Sentinel-2 products (Old version). (2016, January 16). https://www.youtube.com/watch?v=tR_7Oq80TkA