Manual Import of Sentinel-2 Satellite Tiles
Sentinel-2 satellites provide high-resolution multi-spectral images.
Importing satellite imagery can be an excellent alternative to flying over and acquiring data from UAVs in areas where they are not always deployable.
However, it should be noted that the resolution of these images does not equal those that can be acquired by drone.
In the case of the Sentinel-2 tiles import, the Ground Sampling Distance (GSD) is about 10m for 1 pixel.
Thus, some analyses, provided by the platform using images acquired by the drone cannot be performed.
Here is the procedure to follow to download Sentinel-2 satellite images via the Plateform for Exploitation of Sentinel Product (PEPS) and to upload them in Alteia. PEPS is a platform developed by the French National Center for Space Studies (CNES), https://cnes.fr/fr/.
Step 1 - Access to the site: https://peps.cnes.fr/rocket/#/home
Step 2 - Log in to the service or register (if first-time visitor).
Step 1 - Click on “EXPLORER” (top right bar).
Step 2 - Select the period of acquisition of the satellite images.
Step 3 - In“Collection”: Select “SENTINEL-2 tuilés”
Step 4 - Configure the parameters wanted to refine the search (by default, leave these boxes incomplete).
Step 5 - In “Identifiant de tuile MGRS (ex., 31TCJ)”, enter the MGRS tile ID wanted.
To know the MGRS tile ID:
- Go to the website: https://mappingsupport.com/p2/coordinates-mgrs-gissurfer-maps.html
- Preferably select the OpenStreetMap base map in order to obtain an accurate, up-to-date mapping of the entire land surface corresponding to the base map available on the platform.
- Locate the Sentinel 2 tile you want to import.
- Zoom in until obtaining an accurate grid of 100 km square. (For example 31U grid, EQ subgrid).
On the PEPS website:
- Enter this tile identifier number in PEPS.
- (Example:Zone UTM : 31 ; Bande de latitude : U ; Identificateur d'un carré de 100 km :EQ)
Step 6 - In “Couverture nuageuse”, select the percentage of cloud cover wanted.
Note 1: The lower the range, the smaller the selection of downloadable tiles.
Note 2: the cloud cover may be for a part of the tile that you are not interested in, so it is recommended to leave the percentage fairly high. If you want to refine your search, you can select a low cloud cover at this stage.
Step 7 - Click on "RECHERCHER" to launch the search and get access to a drop-down list presenting the different available tiles corresponding to the search criteria.
2.3 Download tile
Step 1 - Select the tile and click on the download button.
Step 2 - If the download button is grayed out, click on add to cart button. The tile can be downloaded later when it is available (quick procedure).
Step 3 - Once the download is complete, extract the compressed file to a well-identified file that will be used later.
To create a reflectance map.
Step 1 - Open the QGIS software:
Step 2 - Create and save a new project.
Step 3 - Click on “Raster” > “Miscellaneous” > “Merge…”
Step 4 - In the tab “Parameters”:
- In “Input layers” click on “…” then “Add File(s)…”.
- Select the previously downloaded (and unzipped) file by double-clicking on it and then opening “GRANULE” and “IMG_DATA”.
- There are 12 different spectral bands acquired by the satellite. There are numbered from 1 to 12, and Select bands from 2 to 8.
- Here is what they correspond to (Wikipedia table, source: https://sentinel.esa.int/web/sentinel/technical-guides/sentinel-2-msi/msi-instrument).
- They are respectively named B02, B03, B04, B05, B06, B07, B08 and are in JP2 format.
- Bands are selected, click on “OPEN” then “OK”.
- Tick “Place each input file into a separate band”.
In the advanced settings:
- Set the number -10000 to “Input pixel value to treat as “no data” [optional]”.
- Set -10000 to “Assign specified “no data” value to output [optional]”.
- Select “No Compression” in the “Profile” line.
- Instead of “NONE” in the “COMPRESS” line, enter the value “LZW”.
- Instead of “IF_NEEDED” in the “BIGTIFF” line, enter the value “YES”.
- On the “Merged” line choose to save or not the reflectance map that will be created when the bands are merged. By default, a temporary file is created. Click on "Run".
Note: The operation may take a few minutes.
- Once the reflectance map is displayed, and ready to be cropped to remove non-desired parts of the map.
Step 5 - With the "Vector" menu Import a vector file (GeoJSON or Shapefile) corresponding to the contours of the parcel.
Note: It's possible to download an existing vector file from Alteia and use it in QGIS.
Once the folder is imported into QGIS, it will be added to the layers.
Step 6 - Right-click on the layer and click on “Zoom to layer”.
Step 7 - Click on “Raster” > “Extraction” > “Clip Raster by Extent…”
Step 8 - In the “Input layer”, select the newly created Reflectance Map.
Step 9 - “Clipping extent” > “[…]” > “Draw on Canvas”
Step 10 - Define the area you want to import to the platform.
2.4.2 Alternative procedure
It is also possible to cut out an area according to a layer to define a precise shape that can follow the shape of a parcel for example.
Step 1 - Enter the value “-10000” in "Assign a specified no data value to output bands[optional]”.
Step 2 - In “Advanced Parameters”, and in “Profile”, select “No Compression” then,
Step 3 - Instead of “NONE” in the “COMPRESS” line, enter the value “LZW”.
Step 4 - Instead of “IF_NEEDED” in the “BIGTIFF” line, enter the value “YES”.
Step 5 - The line “Clipped (extent)”. Choose the location where the newly clipped reflectance map will be saved.
Step 6 - By default, a temporary file is created (download this file anyway to implement it on the Alteia platform).
Step 7 - Click on “Run”.
Step 8 - At the end of this step, you get a raster file corresponding to the cropped reflectance map.
Step 9 - Next step: Integration of the raster on the Alteia platform.
3. Upload in Alteia
Step 1 - To upload a tile in Alteia platform see Manual File Upload and choose to upload this file as a "new survey".
Step 2 - Make sure that the file is in the “Raster” type and in “Category”, select “Reflectances”.
Step 3 - Fill in the names of the corresponding bands.
Step 4 - If the bands have been correctly selected on QGIS, here is how to fill them in on the platform:
- Band Name 1: Blue (B)
- Band Name 2: Green (G)
- Band Name 3: Red (R)
- Band Name 4: Red edge (705 nm)
- Band Name 5: Red edge (740 nm)
- Band Name 6: Red edge (RE)
- Band Name 7: Near IR (NIR)
Step 5 - Click on “UPLOAD”.
Step 6 - The file is downloaded, and different analytics could be launched with Alteia platform as in the examples below.
Example of launching a “Fraction of Vegetation Cover” analysis on a Sentinel-2 raster
RGB colors on the sentinel-2 tile.