Monday, September 13, 2010

Isolating hot spots using Isolines in Saga GIS

In my previous post, I generated a surface grid layer of total incident occurrences where a graduated color sequence indicate areas with the highest density of occurrences i.e. the hot spots in red, as shown in the figure below.

We can further demarcate the hot spots with SAGA GIS's Contour Lines from Grid module.

  1. Start up SAGA GIS and load in the grid layer of the total incident occurrences.
  2. Select Modules | Shapes | Grid | Contour Lines from Grid.

    The Contour Lines from Grid dialog box appears.
  3. In the Grid system drop down list, choose the grid system of the grid layer, e.g. "30; 354x 420y...".
  4. Click the Grid row drop down list and choose the grid layer, e.g. "03.ifelse(lt(a,0),0,a)".
  5. If necessary, change the Minimum Contour Value, Maximum Contour Value and Equidistance values.

  6. Click Okay.

    The contour lines are generated.

    Note: in the Workspace pane, double click the contour line shape line layer to display it overlaid on the incident occurrence grid layer, as shown below.

    Labeling the contour lines
  7. In the Workspace pane, select the Line shape layer of the contour lines e.g. "01.ifelse(lt(a,0),0,a)".
  8. In the Options pane on the right, click the Display:Label Attribute drop down list and choose the attribute field to use for labeling, e.g. "ifelse(lt(a,0),0,a)".

  9. Click Apply.

    The contour lines are labeled.

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