Tuesday, August 17, 2010

Visualize terrain with hill shading in SAGA GIS

By default, SAGA GIS displays grid data with a graduated color table as shown below.

For non-terrain grid data, this is quite okay. When viewing terrain grid data, it is more intuitive to the eye to provide some indications of the 3rd dimension in the planar map view as shown below.

Unlike Global Mapper, a separate hill shade grid layer must be generated explicitly and then overlaid and blended with the terrain layer. This behavior is similar to GeoMedia Grid, another generic grid analysis software. The following paragraphs describe the steps that I did.

Load in a terrain grid file (ESRI ArcGrid ASCII)

  1. Start up SAGA GIS. Select Modules | File | Grid | Import | Import ESRI Arc/Info Grid.

    The Import ESRI Arc/Info Grid dialog box appears.

  2. Click the File row. Click the [...] button on the right.

    The Open dialog box appears.
  3. Browse and select an existing ESRI ArcGrid ASCII file, e.g. elev.asc. Click Open.

    The selected file name is displayed in the Import ESRI Arc/Info Grid dialog box.

  4. Click Okay.

    The selected file is imported into SAGA GIS.
Displaying the elevation grid
  1. Click the Data tab of the Workspace pane on the left.

    The data tree view is displayed.
  2. In the data tree view, select the newly import elevation grid. Right click.

    A pop up menu appears.

  3. Choose Show Grid.

    The elevation grid is displayed in the map view with a default graduated color table.
Generate a hill shaded grid layer
  1. Select Modules | Terrain Analysis | Lighting | Analytical Hillshading.

    The Analytical Hillshading dialog box appears.

  2. Click the Grid system row. In the drop down list, select the newly imported elevation grid system, e.g. 1; 1085x 2699y; 289021.34x 4320943.205000y.
  3. Click the Elevation row. In the drop down list, select the newly imported elevation grid layer, e.g. 01.elev.


  4. Optional. Change the Shading Method, Azimuth, Declination and Exaggeration option values if necessary.
  5. Click Okay.

    The hill shaded grid layer is generated. By default the layer is named 'Analytical Hillshading'
Overlaying and Blending the elevation and hill shaded grid layers
  1. If the Data tree view is not displayed, then click the Data tab in the Workspace pane.
  2. Click the newly created hill shaded grid layer. Right click.

    A pop up menu appears.

  3. Choose Show Grid.

    The Add layer to selected map dialog box appears.

  4. Select the existing map view e.g. 01.elev to append the hill shaded layer together with the elevation grid layer in the same map view. Click OK.

    The hill shaded grid layer is displayed opaquely over the elevation grid layer.

  5. In the Data tree, select the hill shaded grid layer.
  6. If the Settings are not shown in the Options pane on the right, click the Settings tab. In the Transparency field, type in a percentage value e.g. 30.


  7. At the bottom of the right pane, click Apply.

    The hill shaded grid layer is displayed transparently and the underlying elevation grid can be seen.

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