Monday, August 9, 2010

Create intensity images from LAS files with SAGA GIS

In my exploration of SAGA GIS, I tried to create TIFF intensity images from LiDAR LAS files using SAGA GIS. I am pleased with the results and the process, which I shall describe in the following steps below. Like GeoMedia Grid, a LAS file has to be converted into an intermediate Point Cloud format. But the SAGA GIS process seems to have much less overhead than GeoMedia Grid's. Once the LAS file has been converted, the points can be interpolated into a surface using a Delauney Triangulation or other algorithms.

Conversion from LAS to Point Cloud
  1. Startup SAGA GIS.
  2. Select Modules | File | Shapes | Import | Import LAS Files.

    The Import LAS Files dialog box appears.


  3. Select the Input File row. Click the [...] button on the right. Browse and select a LAS file to import. Click Open.
  4. In the Attributes to import besides x,y,z list, toggle on intensity.


  5. Click Okay.

    The LAS file is imported into SAGA GIS.
    Note: it may be necessary to click the Data tab on the Workspace pane on the left to see the newly imported Point Cloud file.


     
Point Cloud Interpolation Using Delauney Triangulation
  1. Select Modules | Grid | Gridding | Interpolation from Points | Triangulation.

    The Triangulation dialog box appears.



  2. In the Points row, click the drop down list and select the previously imported LAS file point cloud data set.
  3. In the Attribute row, click the drop down list and select intensity.




     
  4. Click Okay.

    The User Defined Grid dialog box appears.


  5. Optional. Change any of the options e.g. Cellsize value to 1. Click Okay.

    The interpolated intensity grid is created.


Setting Greyscale colors
  1. In the Data tree, right click on the newly created grid file.

    A pop up menu appears.




  2. Choose Show Grid. Note: The Add layer to selected map dialog box may appear. Click OK.

    The intensity grid is displayed with the default color gradient.




  3. On the Options pane on the right, click the Settings tab. Select the Colors option of the Graduated Color option as shown below.



  4. Click the [...] button.

    The [CAP] Colors dialog box appears.




  5. Click the Presets button. Choose the Greyscale in the Preset Selection dialog box. Click OK.

    The [CAP] Colors dialog box is updated.




  6. Click Okay.



  7. Click the Apply button on the Options pane.

    The intensity grid display is updated with the greyscale color range.


Saving as TIFF
  1. In the Data tree, right click on the intensity grid.

    A pop up menu appears.


  2. Choose Save Grid as Image.

    The Save As Image dialog box appears.



  3. Browse to an output folder. Type in a file name. Choose Tagged Image File Format in the Save as type drop down list. Click Save.

    The Save Grid as Image dialog box appears.
    Note: toggle on Save Georeference to create the *.tfw world file. Toggle on Legend: Save to create the *_legend.tif file of the color legend.



  4. Click Okay.

    The intensity TIFF file is created.


    Below is an example of the resultant legend TIFF
    file.




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