Wednesday, February 3, 2010

Create donut polygons in Global Mapper

Global Mapper has a neat function to create donut polygons. It can be useful if you have some islands in closed water bodies like lakes and you need to retain the island polygons while cutting out holes from the enclosing water body polygons. Here's how to use the function.
  1. Run Global Mapper and display or digitize a water body polygon.


  2. Press ALT+D or click the Digitizer Tool icon. Then click the Create New Area Feature  icon.

    The cursor changes to an arrow with the label Area.
  3. Click on the map view to place an island polygon.Right click to complete.

    The Modify Feature Info dialog box appears.

  4. Change the Feature Type to island if needed. Change the Feature Layer if necessary. Click OK.

    The island polygon is created.
  5. Press ALT+D or click the Digitizer Tool  icon.

    The cursor changes to a cross hair with the label Edit.
  6. Click on the newly created island polygon.

    The island polygon is selected.

  7. Right click anywhere on the map view.

    A pop up menu appears.

  8. Choose Cut Selected Area(s) from Another Area (Add Islands).

    The cursor changes to an arrow with the label Parent.
     
  9. Click on the enclosing water body polygon.

    The message below appears.

  10. Click Yes.

    Note: If you want to keep the island polygon as a separate feature, then click No. Otherwise, if you click Yes, the command will delete the island polygon after cutting out the donut hole.

    The donut polygon is created.

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