fGIS Polygon Editing Tutorial - (Updated for 2005.01.12 build of fGIS)

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Index:

 

A.  Splitting Polygons using the Split Shape Tool

B.  Appending Polygons using the Subtract Polygon Command

C.  Creating Holes in Polygons and Removing Holes from Polygons

D.  Drilling Polygons using the Drill Polygon Command

E.   Creating Polygons that are Within other Polygons

 A.  Splitting Polygons using the Split Shape Tool

 The Split Shape Tool () is used to split existing polygons into smaller polygons. 

In the following example, the green polygon labeled "PR" (show below) will be split into two polygons using the Split Shape Tool.

 

The green polygon labeled "PR" will be split using the Split Shape Tool. 

1.  Select the Edit - Start Editing command to edit the polygon shapefile.

2.  Click on the Split Shape Tool ().

3.  Digitize a split line that begins and ends outside of the polygon to be split, as shown below.

 

To split a polygon, the split line must begin and end outside of the polygon. 

4.  Double-click to end the split line and split the polygon into two polygons.

5. The result of splitting the polygon with the Split Shape Tool is shown below.  Notice how the attributes of the original polygon have been copied to the newly created polygons and that the label is automatically updated.

 

The results of splitting a polygon using the Split Shape Tool. 

B.  Appending Polygons using the Subtract Polygon Command

When digitizing polygons that are adjacent to one another, it is often easier to append a new polygon using the Subtract Polygon command, rather than trying to snap the new vertices to the vertices of an existing polygon.

In the following example, the red polygon labeled "A" (shown below) will be appended to the adjacent polygons using the Subtract Polygon command.

 

The red polygon labeled "A" will be appended using the Subtract Polygon command.

1.  Select the Edit - Start Editing command to edit the polygon shapefile.

2.  Using the New Shape Tool (), digitize a new polygon that overlaps the edges of the green and blue polygons as shown below.

 

The overlapping polygon will be "subtracted" from the polygons that it overlaps. 

3.  Select the overlapping polygon using the Pick tool ().

4.  Select the Edit - Polygon Functions - Subtract Polygon command to "subtract" the overlapping polygon from the polygons that it overlaps.

5.  The result of "subtracting" the polygon with the Subtract Polygon command is shown below.  The boundaries of the red polygon labeled "A" are now coincident with the boundaries of the adjacent polygons.

 

The boundaries of the red polygon labeled "A" are now coincident with the boundaries of the adjacent polygons. 

C.  Creating Holes in Polygons and Removing Holes from Polygons

 Often it is necessary to create holes in polygons or remove holes from polygons, like the hole shown in the green polygon labeled "PR" shown below.

 

The green polygon labeled "PR" contains a hole.

 

To add a hole to a polygon, use the following procedure.

1.  Select the Edit - Start Editing command to edit the polygon shapefile.

2.  Select the polygon to add a hole to using the Edit Points/Edit Select tool ().  Note:  The polygon is selected when each of its vertices are numbered.

3.  Right mouse click on the selected polygon where you want to start adding the hole and select the Polygon Functions - Add Part (or make hole) command, as shown below.  Note:  The Add Part (or make hole) command is only available on the right mouse click popup menu and NOT on the Edit menu.

 

The Add Part (or make hole) command is used to make holes in polygons.

 

4.  Now digitize the boundary of the hole.  Note:  It's best to digitize a hole in the clockwise direction, otherwise the polygon will have to be fixed using the Polygon Functions - Fix Polygon Winding command.

5.  To finish digitizing the hole, double-click OR select the Edit - Close Shape command.

6.  If the hole doesn't appear when it is digitized, make sure the polygon is still selected and select the Polygon Functions - Fix Polygon Winding command.

 To remove a hole from a polygon, use the following procedure.

1.  Select the Edit - Start Editing command to edit the polygon shapefile.

2.  Select the border of the hole to be removed using the Edit Points/Edit Select tool ().  Note:  The hole is selected when each of its vertices are numbered and shown in green as shown below.

 

The hole in the green polygon labeled "PR" is selected when its vertices are numbered and shown in green.

3.  Once the hole has been selected, select the Polygon Functions - Delete Part command to remove the hole from the polygon.

  D.  Drilling Polygons using the Drill Polygon Command

 When digitizing polygons into areas where polygons already exist, it is often easier to "drill" a new polygon using the Drill Polygon command, rather than trying to snap the new vertices to the vertices of an existing polygon.

Note: The Drill Polygon command only works with polygons that intersect the borders of polygons they overlap.  To "drill" a polygon that is entirely contained within another polygon, first create a hole in the containing polygon, then "snap" a new polygon into the hole.

In the following example, the red polygon labeled "A" (shown below) will be "drilled" into the adjacent polygons using the Drill Polygon command.

 

The red polygon labeled "A" will be drilled into the adjacent polygons using the Drill Polygon command.

1.  Select the Edit - Start Editing command to edit the polygon shapefile.

2.  Using the New Shape Tool (), digitize a new polygon that overlaps the edges of the green, blue, brown, and peach polygons as shown below. 

3.  Select the new shape using the Pick tool ().  Note: If the new shape overlaps other shapes entirely (as it does in this example), then use the Layer - Select Last Shape command to select the new shape.

The overlapping polygon will be "drilled" into the polygons that it overlaps.

 4.  Select the Edit - Polygon Functions - Drill Polygon command to "drill" the overlapping polygon into the polygons that it overlaps.

5.  The result of "drilling" the polygon with the Drill Polygon command is shown below.  The boundaries of the red polygon labeled "A" are now coincident with the boundaries of the adjacent polygons.

 

The boundaries of the red polygon labeled "A" are now coincident with the boundaries of the adjacent polygons.

 

E. Creating Polygons that are Within other Polygons

 As mentioned above, the Drill Polygon command can NOT be used to create polygons that are entirely within the boundaries of another polygon.

To create polygons that are entirely within other polygons, use the following procedure.

1.  First create a hole in the underlying polygon that is the shape of the polygon you want to create using the procedure mentioned in C. Creating Holes in Polygons and Removing Holes from Polygons (above), as shown below.

 

First create a hole in the underlying polygon.

2.  Set the Snapping layer to the editable layer using the snap layer listbox, shown below.

 

Set the Snapping layer in the snap layer listbox.

 

3.  Then, using the New Shape Tool (), digitize a new polygon and "snap" its vertices into the newly created hole as shown below.

 

The new polygon is being "snapped" into the hole.

 (Brian Brown, 1/05)

 

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