The Thiessen polygon generator requires a boundary to be specified around the area in which Thiessen Polygons will be created. This is to prevent the outside edge of the polygons along the perimeter of this area from extending to infinity. The generator can automatically create a boundary using the Buffering Point Area Percentage value, or it can use a previously created polygon feature class as the boundary.

A border polygon feature class can be created in ArcCatalog, and edited in ArcMap.

To create a border feature class, you will need a Bentley StormCAD CONNECT model that has had at least one scenario published as an Esri feature dataset. Then, follow these steps:

1. In the directory structure pane of ArcCatalog, right-click the Bentley StormCAD CONNECT feature dataset and select New...Feature Class.
2. A dialog box will open, prompting you to name the new feature class. Enter a name and click Next.
3. In the second step, you are prompted to select the database storage configuration. Do so, and click Next.
4. In the third step, click the Shape cell under the Field Name column, and ensure that the Geometry Type is Polygon. Click Finish.
5. In ArcMap, click the Add Data button and select your Bentley StormCAD CONNECT feature dataset.
6. Click the Editor button and select Start Editing. Ensure that the border feature class is selected in the Target drop-down list.
7. Draw a polygon around the point features that you wish to be used to generate the polygons. When you are finished drawing the polygon, click Editor...Stop Editing. Choose Yes when prompted to save your edits.

The polygon feature class you just created can now be used as the boundary during Thiessen polygon generation. For more information about creating and editing feature classes, see your ArcGIS documentation.