SAR flood mapping with SNAP

In this tutorial we will use Level-1 Ground Range Detected (GRD) Sentinel-1 data downloaded from the Copernicus Open Access Hub. Download the example data from Stud.IP. It is from a study area in Nigeria near the city of Lokoja in Kogi province near the junction of the two large rivers Niger and Benue. During the westafrican monsoon April - September heavy rainfall and floods occur.

Data transfer

Download example tutorial data from Nigeria or data from your personal region of interest: two Sentinel-1 scenes one pre-event acquisition date and one post-event date: Downloading Sentinel-2 images. Select Product type GRD with VV polarization.

Visualization of amplitude bands

• Open SNAP Desktop.
• Drag and drop the zip files of the Sentinel-1 scenes into the TOC of Product Explorer.
• Unfold the product folders and Mark the Aplitude_VV bands of both products.
• Right click Open 2 Image Window.
• Windows --> Tile Horizontally. Both Amplitude bands are shown side by side geometrically linked to each other. Make sure the option View --> Synchronise Image View and View --> Synchronise Image Cursorsis activated).

Subsetting

We need to reduce the data volume by subsetting the large Sentinel-1 scenes.

• Activate the Rectangle Drawing Tool
• Left click and drag a rectangular box in the viewer to mark a subset area.
• Activate the selection tool and mark the box in the viewer.
• Right click WKT from Geometry.
• Copy the WKT string: CTRL + C.

• Open Tools --> GraphBuilder.
• Click Load and browse to the graph file \geodata_lokoja\01_lokoja_subset.xml'
• Click on the Subset tab and activate Geographic Coordinates.
• Paste the WKT string into the text field below the world map: CTRL + V.
• Check if the extent of the subset is displayed correctly onto the world map.
• Click Update to apply the box coordinates.

• Click the Write tab. Adjust the output directory to you local path.
• Click Run.
• Click the Read tab.
• Choose the pre-event Sentinel-1 source product (2018-04-01) from the drop down list.
• Click Run.
• Repeat the worksteps above creating a subset for the post-event image (2019-09-16).

Radiometric and geometric calibration

File --> Close all products from previous steps. File --> Open product. Browse to subset files resulting from previous work step with prefix Subset_ and extension _Orb.dim (pre-event and post-event subsets).

• Open Tools --> GraphBuilder.
• Click Load and browse to the graph file \geodata_lokoja\02_lokoja_calibration.xml'.

• Click the Read tab.
• Choose the pre-event Sentinel-1 source product with prefix Subset_ and extension _Orb.
• Click Run.
• Click the Write tab. Adjust the output directory to you local path. Make sure that the file name matches the input pre-event product.
• Click Run.
• Repeat the same for the post-event Sentinel-1 subset. Make sure that file names match for reading and writing!

Change composite

• Click Load and browse to the graph file \geodata_lokoja\03_lokoja_change_composite.xml.

• Click the Write tab. Adjust the output directory to you local path.
• Run.
• Open QGIS.
• Data Source Manger --> Browser --> XYZ Tiles --> Google Satellite
• Load the change compoisite file with extension _Stack.tif Data Source Manger --> Raster.
• Adjust opacity of the change layer on top of Google Satellite. Layer Properties --> Transparency --> Global Transparency.