Monday, October 11, 2010

Mosaic DEM grid layers using SAGA GIS

If an area of interest is covered by multiple digital elevation (DEM) model files, then it may be necessary to mosaic or merge the individual DEM files to create a single DEM grid layer. The Grid Merging command in SAGA GIS can be used to perform this task. 

In this example, I will illustrate importing and merging a few SDTS DEM layers. 

Importing SDTS DEM files
  1. Start up SAGA GIS.
  2. Select Modules | File | GDAL/OGR | GDAL: Import Raster.

    The GDAL: Import Raster dialog box appears.

  3. Select the Files row. Click the Browse [...] button.

    The Open dialog box appears.

  4. Browse and select a SDTS DEM file, e.g. 9903CATD.DDF. Click Open.

    The GDAL: Import Raster dialog box is updated with the selected file.
  5. Click Okay.

    The SDTS DEM layer is imported.
  6. Repeat steps 2 to 5 for importing the other DEM files of interest.

Displaying DEM files in one Map window
  1. In the Workspace pane on the left, double click one of the imported DEM layer.

    The layer is displayed in a Map window.
  2. Double click on another grid layer.

    The Add layer to selected map dialog box appears.

  3. Choose the previously created Map window instead of New. Click OK.

    The layer is displayed in the same Map window.
  4. Repeat 2 to 3 for the remaining grid layers.
  5. Select Map | Zoom to Full Extent.

    All the grid layers can be seen in the same Map window.
Merge the DEM layers into one
  1. Select Modules | Grid | Construction | Merging.

    The Merging dialog box appears.

  2. Select the Grids to Merge row. Click Browse [...].

    The Grids to Merge dialog box appears.

  3. On the list on the left, select all the grid layers to merge (while pressing down the CTRL button). Click the >> button.


  4. Click Okay.


  5. In the Target Grid combo box, choose create.
  6. If necessary, change the Preferred data storage type, Interpolation, or Overlapping Cells options.
  7. Click Okay.

    The selected grid layers are merged.

  8. In the workspace pane, double click on the merged grid and display in a new Map window.


7 comments:

Jorge Santos said...

Very nice tutorial!

My name is Jorge and I have a blog in Portuguese with posts about geoprocessing tips and free GIS. Your job is great man! Dude, you have a good knowledge about raster process, I envy you, rs,

Best Regards,

Jorge Santos
http://processamentodigital.blogspot.com/

d0min0c said...

Jorge, you have a good blog yourself!

Ale said...

hello Domino,
thank you for your useful tutorial.
I am beginning to use SAGA and I like it.
By the way, something has changed in SAGA 2.0.8 Merge module:
1) it is now in "grid - tools - merging"
2)when you create the new grid, in the Grid system combo box choose not set and in the Target Grid combo box, choose not set. (if you coose "crete", Saga will process only the first grid.
ciao!
Ale

d0min0c said...

Thanks for the update Ale. But this behavior looks less intuitive than before.

LCL said...

Thanks Domino,
This post help me a lot with my research work.
Best wishes

Leo

LCL said...

Domino, thanks for this post.
It help me a lot in my current research work with basins.
By the way, do you have another post about how do get a shape file from a grid file?
I am working with the LS factor grid (from SAGA GIS) but I can find a way to get a shape file from a LS grid with a table with the respective LS values of each polygon.

dominoc said...

Hi LCL
I don't have but you should be able to use the Vectorising Grid Classes module under the Shapes - Grid category for your conversion needs.

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