Monday, January 31, 2011

Determine the highest (or lowest) elevation point within a polygon with SAGA GIS

The example workflow described here can be used to locate possible sites for locating radio towers or something similar. Along the way, it also illustrates how to use SAGA GIS to create a buffer zone around a point, how to use the buffer zone as a polygon to find the location of the maximum elevation on a DEM, and how to sort attribute tables.

Load in a DEM grid layer and a point shape layer
  1. Start up SAGA GIS and load in and display a DEM grid layer, e.g. ground.asc.
  2. Load in and display a point shape file, e.g. bufferPoint.shp in the same map window.

    The grid layer and point shape layer are loaded.


Create the buffer zone

  1.  Select Modules | Shapes | Tools | Shapes Buffer.

    The Shapes Buffer dialog box appears.
  2. Click the Shapes row and select the point shape layer e.g. 01.bufferPoint.
  3. In the Buffer Distance (Fixed) field, type in a distance, e.g. 100.

  4. Click Okay.

    The buffer polygon is created around the point.
Generating the highest (and lowest) elevations for the entire DEM grid layer
  1. Select Modules | Shapes | Grid | Local Minima and Maxima.

    The Local Minima and Maxima dialog box appears.
  2. Click the Grid system row and choose the DEM grid system, e.g. 1;684x684y;312480x5195216y.
  3. Click the Grid row and choose the input DEM grid layer, e.g. 01.ground.

  4. Click Okay.

    The local minima and maxima point shapes for the entire DEM grid layer are created.
Finding only the local maxima point shapes within the buffer
  1. Select Modules | Shapes | Construction | Cut Shapes Layer.

    The Cut Shapes Layer dialog box appears.
  2. Click the Shapes row. Then click the browse [...] button.

    The Shapes dialog box appears.
  3. Select the local maxima point shape layer, e.g. 03.ground[Local Maxima] on the left list. Click the >> button.

    The selected layer is shifted to the right list.
  4. Click Okay.
  5. Ensure the Method is completely contained. Click the Options Extent row and select polygons.

  6. Click Okay.

    Processing messages appear.


    The Polygons dialog box appears
    .
  7. Click the Polygons row and choose the buffer shape layer, e.g. 01.bufferPoint[Buffer].

  8. Click Okay.

    The local maxima points within the buffer is generated.
Locating the maximum point within the buffer
  1. In the Workspace pane, mouse right click on the local maximum points within the buffer layer, e.g. 04.ground[Local Maxima][Cut].

    A pop up menu appears.
  2. Select Attributes |  Show Table.

    The attribute table appears.
  3. Select Window | Tile Horizontally.

  4. In the attribute window, double click the Z column title header to sort the records so that the maximum Z value is the first row.
  5. Select the first row (which has the maximum elevation).

    The corresponding point on the map window is highlighted.

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