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Making "holes" in polygons

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Explicitly, Easy Trace Pro dos not support multiply connected polygons i.e. ones, described by several disconnected boundaries. But it does not mean that there is no way to create polygons with holes.

Explicitly, Easy Trace Pro dos not support multiply connected polygons i.e. ones, described by several disconnected boundaries. It does not mean however that there is no way to create polygons with holes, although you should remember the following:

  1. The outer contour and the hole are connected by a bridge and form a single closed polyline. The bridge is a passage from the contour to the hole and back.
  2. Every hole is connected with the outer contour either directly or through a neighboring hole (in that case, there is a bridge between the holes).
  3. The bridges disappear at export to systems that support multiply connected polygons (ArcGis, Mapinfo, etc.)
  4. Export does not delete bridges with misalignment of vertices in the “there” and “back” branches.
How to make holes in a polygon? You need two polygons at the least, one completely inside the other. The polygons may be attributed to the same or different layers, but the layers should be polygonal (marked with red rhombs in the Layer Manager dialog box).

METHOD ONE

Available beginning from Version 7.7.

  1. Select the outer polygon with the Editor (arrow).
  2. Press the mouse right button and select Scissors (or hot key W to start this mode).
  3. Specify any point INSIDE the polygon you want to cut out from the bigger one. That’s all. Specifying of the polygon boundary by mistake does not cause much harm – you will just dissect it...

METHOD TWO

Available for users of later versions.

  1. Select the Shears tool (Alt+W).
  2. Hold Ctrl pressed (the cursor turns into shears with an arrow), and specify all polygons you want to cut out.
  3. Press the mouse right button and select "Split & Delete".
The tool affects not polygons only but all visible objects. Beginning from Version 8.5, it is possible to select the polygons to be cut out with the Group Editor first and then to apply Shears.

Selected polygons should not overlap or be embedded otherwise they will cut each other. The tool uses copies of polygon boundaries (rather than polygons themselves) as cutting edges and thus it can dissect the polygons you have selected just as any other object. To avoid it for sure, freeze the layer of polygons you want to cut out.

Beginning from late builds of Version 8.5, it is possible to select layers for the operation in the Parameters Bar of Shears. The tool does not affect layers that are not specified.

METHOD THREE

Available beginning from Version 8.6

  1. Run the Polygon Subtraction utility and specify the layers of minuend and subtrahend polygons.  For example, you may subtract islands (subtrahend polygons) from lakes (minuend polygons).
  2. It is also possible to use the Create Polygons utility. Switch on the “Cut out internal polygons” option at polygon forming and the program will make the holes automatically.
  3. The Autodetect Lakes utility may also be useful. Embedded polygons generated at polygon forming after automatic digitization may be cut out from the outer polygons at once.
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