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CCAD's AutoCAD Tutorials

Regions

Regions are 2 dimensional shapes created from closed shapes or loops. Regions are helpful when calculating areas, perimeters, centroids and other mass properties. Regions may be created using the REGION command or the BOUNDARY command.

The Region Command

To create a Region from a closed shape, like the Spline comprising the POND in the illustration below, select the REGION command from the Draw menu or toolbar.

creating a region

The prompt, Select objects: is displayed. Pick the objects or group of objects comprising the closed shape and press ENTER. The selected objects must be coplanar, end-connected curves and lines.

The original Spline object is deleted unless the DELOBJ system variable is set to 0.

To determine the mass properties of the newly created Region, select the MASS PROPERTIES command from the Inquiry menu or toolbar. The prompt, Select objects: is displayed. Pick the Region and press ENTER.

Answering Yes to the final prompt will create a file of the Mass Property data for the Region.

region properties file screen shot


The Boundary Command

Another method of creating regions is to use the BOUNDARY command, which is very similar to the BOUNDARY HATCH command, except that it creates only a boundary and does not fill the closed area.

The areas shown below are not end-connected curves and lines; therefore the BOUNDARY command should be used to create the three regions.

Region creation

The POND will also be a Region, because it is wholly contained in the larger surrounding region.

To create Polylines or Regions, select the BOUNDARY command from the Draw menu.

Boundary Creation dialog box

The Object type drop down (circled above) determines whether Polylines or Regions are created with the BOUNDARY command.

The Pick Points button allows you to select points within a closed shape.

boundaries

After selecting internal points 1 and 2 and pressing ENTER, the dotted boundaries shown above will be turned int Regions (in red, below).

regions


The Regions created by the BOUNDARY command are separate regions. However, they can be combined with the Subtraction, Union or Intersection commands. For instance, to determine the area of the property not occupied by water or buildings, the two larger Regions need to be joined and the POND Region subtracted.

Select the SUBTRACT command from the Modify, Solids Editing menu.

subtracting regions

Select Regions 1 and 2 as the Regions to Subtract from. Press ENTER to select Region 3 to be subracted. Press ENTER to complete the command.

To determine the mass properties of the newly created Region, select the Mass Properties command from the Inquiry menu or toolbar. The prompt, Select Objects: is displayed. Pick the Region and press ENTER.

new regions properties file

The BOUNDARY command is also useful in creating polylines used as profiles in 3D Solids commmands.