The Features | New Features | Union of Polygons command automatically creates a new polygon by combining multiple objects. This command can be used with any combination of existing polygons, rectangles, rounded rectangles, and/or ellipses. The original objects are retained or removed based on the Keep original objects selection in the Union Polygons dialog.
Selections including objects that do not touch or intersect result in a complex polygon, similar to the Combine Island/Lakes command when combining islands.
Sometimes selections made up of many contiguous objects, for example states or provinces in a country, have objects that do not completely touch or intersect. If objects do not touch or intersect along the entire border, the Union Polygons command results in a complex polygon with small lakes. To remove the lakes, use the Split Islands/Lakes command, and then delete the interior polygons. Alternatively, use the Reshape command to delete the vertices for the lakes.
To use the Union Polygons command
- Select all the objects that you wish to combine into a single polygon.
- Click the Features | New Features | Union of Polygons command.
- Select whether or not to keep the original objects in the Union Polygons dialog.
- Click OK in the Union Polygons dialog, and the selected objects are combined into a single polygon. The new polygon is selected when the process is completed.
With Union Polygons, you do not have to preserve the original objects. In this example, four new polygons are based on groups of counties. When creating the example on the left, Keep original objects was not checked. Keep original objects was checked when creating the example on the right.
Difference between Combine Islands/Lakes and Union Polygons
This command differs from the Combine Islands/Lakes command. With the Union Polygons command, a new polygon is created from the selected objects. With Combine Islands/Lakes, the selection is combined into a single complex polygon. The original objects become islands and lakes, and a new polygon is not created.
Contiguous objects are part of the created polygon with the Union Polygons command. With the Combine Islands/Lakes command, contiguous objects are separate islands in a single complex polygon.
Intersecting areas of the selected objects are treated as part of the new polygon when the Union Polygons command is used. When the Combine Islands/Lakes command is used, intersecting portions of the selection are treated as lakes, or holes, in the complex polygon.
The Combine Islands/Lakes and Union Polygons commands behave similarly only when objects in the selection do NOT touch or intersect. However, the Combine Islands/Lakes command combines the selection into a complex polygon. The Union Polygons command creates a new complex polygon, and the original objects can be removed or kept.
For example, if you want to indicate an outer boundary with a heavy line and preserve the inner boundaries with thinner lines as in the example above, you can accomplish this with the Union Polygons command, not with the Combine Islands/Lakes command.