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 April - September 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.  
  
 
=Data transfer=
 
=Data transfer=
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=Visualization of amplitude bands=
 
=Visualization of amplitude bands=
 
* Open SNAP Desktop.  
 
* Open SNAP Desktop.  
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* Drag and drop the zip files of the Sentinel-2 scenes into the TOC of Product Explorer.
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* 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.
 
* Unfold the product folders and Mark the Aplitude_VV bands of both products.
 
* Right click '''Open 2 Image Window'''.
 
* 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. The geometry ist still in ascending sensor coordinates.  
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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_sidebyside.png|800px]]
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[[File:snap_dual_view.png|800px]]
  
 
=Subsetting=  
 
=Subsetting=  
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Open SNAP Desktop.
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We need to reduce the data volume by subsetting the large Sentinel-1 scenes.
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Drag and drop the zip files of the Sentinel-2 scenes into the TOC.
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* Activate the Rectangle Drawing Tool [[File:snap_box_drawing.png]]
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We need to reduce the amount data by subsetting the large Sentinel-1 scenes.
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* Left click and drag a rectangular box in the viewer to mark a subset area.
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Zoom out and activate the Rectangel Drawing Tool [[File:snap_box_drawing.png]]
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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).
  
 
=Radiometric and geometric calibration=
 
=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=
 
=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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