SAR flood mapping with SNAP

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In this tutorial we will use Level-1 Ground Range Detected (GRD) Sentinel-1 data downloaded from the Copernicus Open Access Hub. The example data 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 heavy rainfall and floods occur.  
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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.  
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= Data transfer =
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Download example tutorial data from Nigeria or data from your personal region of interest: two Sentinel-1 scenes one acquisition pre-event and one date post-event: [[Downloading Sentinel-2 images]].
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=Data transfer=
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= Subsetting =  
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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]].
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Open SNAP Desktop.  
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Select Product type '''GRD''' with '''VV''' polarization.
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Drag and drop the zip files of the Sentinel-2 scenes into the TOC.  
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We need to reduce the amount data by subsetting the large Sentinel-1 scenes.
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[[File:esa_openhub_S1.png|600px]]
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= Radiometric anfd geometric calibration =
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=Visualization of amplitude bands=
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= Stacking and  
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* Open SNAP Desktop.  
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* Drag and drop the zip files of the Sentinel-1 scenes into the TOC of Product Explorer.
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* Unfold the product folders and Mark the Aplitude_VV bands of both products.
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* Right click '''Open 2 Image Window'''.
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* {{mitem|text=Windows --> Tile Horizontally}}. Both Amplitude bands are shown side by side geometrically linked to each other. Make sure the option {{mitem|text=View --> Synchronise Image View}} and  {{mitem|text=View --> Synchronise Image Cursors}}is activated).
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[[File:snap_dual_view.png|800px]]
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=Subsetting=
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We need to reduce the data volume by subsetting the large Sentinel-1 scenes.
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* Activate the Rectangle Drawing Tool [[File:snap_box_drawing.png]]
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* Left click and drag a rectangular box in the viewer to mark a subset area.
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* Activate the selection tool [[File:snap_selection_tool.png]] and mark the box in the viewer.
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* Right click '''WKT from Geometry'''.
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* Copy the WKT string: CTRL + C.
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[[File:snap_wkt_geometry.png|600px]]
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* Open {{mitem|text=Tools --> GraphBuilder}}.
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* Click {{button|text=Load}} and browse to the graph file ''\geodata_lokoja\01_lokoja_subset.xml'''
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* Click on the '''Subset''' tab and activate '''Geographic Coordinates'''.
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* Paste the WKT string into the text field '''below''' the world map: CTRL + V.
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* Check if the extent of the subset is displayed correctly onto the world map.
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* Click {{button|text=Update}} to apply the box coordinates.
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[[File:snap_subset_graph.png|600px]]
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* Click the '''Write''' tab. Adjust the output directory to you local path.
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* Click {{button|text=Run}}.
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* Click the '''Read''' tab.
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* Choose the pre-event Sentinel-1 source product (2018-04-01) from the drop down list.
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* Click {{button|text=Run}}.
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* Repeat the worksteps above creating a subset for the post-event image (2019-09-16).
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=Radiometric and geometric calibration=
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{{mitem|text=File --> Close all products}} from previous steps.
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{{mitem|text=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).
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* Open {{mitem|text=Tools --> GraphBuilder}}.
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* Click {{button|text=Load}} and browse to the graph file ''\geodata_lokoja\02_lokoja_calibration.xml'''.
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[[File:snap_graph_calibration.png|600px]]
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* Click the '''Read''' tab.
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* Choose the pre-event Sentinel-1 source product  with prefix '''Subset_''' and extension '''_Orb'''.
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* Click {{button|text=Run}}.
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* Click the '''Write''' tab. Adjust the output directory to you local path. Make sure that the file name matches the input pre-event product.
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* Click {{button|text=Run}}.
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* Repeat the same for the post-event Sentinel-1 subset. Make sure that file names match for reading and writing!
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=Change composite=
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* Click {{button|text=Load}} and browse to the graph file ''\geodata_lokoja\03_lokoja_change_composite.xml''.
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[[File:snap_graph_composite.png|600px]]
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* Click the '''Write''' tab. Adjust the output directory to you local path.
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* {{button|text=Run}}.
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* Open QGIS.
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* {{mitem|text=Data Source Manger --> Browser --> XYZ Tiles --> Google Satellite}}
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* Load the change compoisite file  with extension '''_Stack.tif''' {{mitem|text=Data Source Manger --> Raster}}.
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* Adjust opacity of the change layer on top of Google Satellite. {{mitem|text=Layer Properties --> Transparency --> Global Transparency}}.
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[[File:snap_change_composite.png|600px]]
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[[Category:SNAP Tutorial]]
 
[[Category:SNAP Tutorial]]
 
[[Category:Tutorials]]
 
[[Category:Tutorials]]

Latest revision as of 14:06, 25 January 2021

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.

Contents

[edit] 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.

Esa openhub S1.png

[edit] 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).

Snap dual view.png

[edit] Subsetting

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

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

Snap wkt geometry.png

  • 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.

Snap subset graph.png

  • 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).

[edit] 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'.

Snap graph calibration.png

  • 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!

[edit] Change composite

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

Snap graph composite.png

  • 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.

Snap change composite.png

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