Union of Polygons

The Boundary | New Boundaries | Union of Polygons command is a way to automatically trace around the outside of a group of contiguous areas. This command is useful for defining regions based on existing areas. For example, you might have a sales territory based on several counties. If you want to indicate the sales territory on the map, select all the counties that define that territory and choose the Union of Areas command. This traces around the outside of the entire group. You can apply a unique line style to the new traced boundary for emphasis, or you can assign a primary ID to the new area so you can link it to data.

To use the Union of Polygons command:

1. Select all the areas that define the boundary to be grouped together.

2. Choose the Boundary | New Boundaries | Union of Polygons command.

3. Select options for the original objects and lakes in the Union of Areas dialog.

4. Click the OK button and the selected areas are traced. The new traced boundary is the selected area when the process is completed.

With Union of Areas, you do not have to preserve the original boundaries. In this example, four new

boundaries are based on groups of counties. When creating the example on the left, Keep original areas

was not checked. Keep original areas was checked when creating the example on the right.

Union of Areas Dialog

The Union of Areas dialog contains the options for the Union of Polygons command.

Select whether to keep original areas and/or

inner lakes with the Union of Areas dialog

Keep Original Areas

When Keep original areas is activated, the selected areas are traced to create a new area while leaving the original selected areas unchanged. When Keep original areas is NOT activated, the areas are traced to create a new area, and the original group is deleted.

Keep Inner Lakes

When Keep inner lakes is activated, inner boundaries, called lakes, are included in the new traced boundary. When Keep inner lakes is not activated, inner boundaries, called lakes, are removed from the new traced boundary.

Difference between Combine Islands/Lakes and Union of Areas

This command differs from the Combine Islands/Lakes command. With the Union of Areas command, the outside boundary for the selected group is generated. With Boundary | Combine Islands/Lakes, the group is combined into a single complex polygon, but the outer boundary is not traced. If you want to indicate the outer boundary with a heavy line and preserve the inner boundaries with thinner lines, you can accomplish this with the Union of Polygons command, not with the Combine Islands command.