Monday, January 3, 2011

How to digitize a new shape object in SAGA GIS

There are commands to create a new shape layer, create and edit a shape object in SAGA GIS but it is not very obvious how to use them. It took me a little while to figure out how to use the commands given that there wasn't a lot of information about it. In order to digitize a new shape with reference to a backdrop, the shape layer must first be displayed in the same map window as the reference layer(s). Only within this context will the shape layer commands be available and functional.

Create a new Shape layer
  1. Start up SAGA GIS and load in a reference file e.g. a DEM file, and open it in a new map window.


  2. Select Modules | Shapes | Construction | Create New Shapes Layer.

    The Create New Shapes Layer dialog box appears.

  3. Optional. Click the Name row and type in a new shape layer name.
  4. In the Shape Type field, choose a geometry type, e.g. Polygon.
  5. Optional. In the Number of Attributes field, type in the number of attribute fields the shape layer will have, e.g. 2.
  6. Select the Attributes row. Click the [...] button.

    The Attributes dialog box appears.

  7. If necessary, change the Name and Type for each attribute field. Click Okay.
  8. Click Okay to close the Create New Shapes Layer dialog box.

    An empty shapes layer is created.

  9. In the Workspace pane's Data tab, double click on the newly created shapes layer.

    The Add layer to selected map dialog box appears.
  10. In the Map Selection list box, choose an existing map with the reference layer(s) instead of New. Click OK.
Adding a New Shape
  1. In the Workspace pane, right click on the newly created shape layer.

    A pop up menu appears.

  2. In the pop up menu, choose Edit | Add Shape.
  3. Click the Action button  on the tool bar.

    The cursor changes to a cross hair with the label i.
  4. Mouse left click a few points to digitize the shape geometry.


  5. Right mouse click to complete the digitizing.

Remove an existing vertex
  1. Move the cursor until it hovers over an existing vertex.

    The cursor changes to a cross hair with a dot circle label.
  2. Mouse left click on the vertex.

    The vertex is selected in red.
  3. Press the right button of the mouse.

    A pop up menu appears.

  4. Choose Delete Selected Point.

    The point is deleted.
Adding a new vertex to an existing shape
  1. Move the cursor until it hovers over the location on the existing line work of the shape object.

    The cursor changes to a cross hair with a plus sign.

  2. Press down the left mouse button and drag the cursor to the new vertex location.

    A new vertex is inserted.
Creating a hole in an existing polygon
  1. Mouse right click.

    A pop up menu appears.

  2. Choose Add Part.
  3. Click the mouse left button and digitize a few vertices inside the polygon. Click the mouse right button to complete.

    A hole is created.

Complete the shape object
  1. Mouse right click anywhere in the map window.

    A pop up menu appears.

  2. Toggle off Edit Selected Shape.

    The Save changes prompt appears.

  3. Click Yes.

    The new shape object is saved into the shape layer.

1 comment:

Evan said...

Thank you!

Its much appreciated your share

Keep mapping!

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