Fixed Elevation Topography
If you don’t have any data suitable for creating a topography, you can set a fixed elevation. To do this, right-click on the Topographies folder and select New Topography > Fixed Elevation. In the window that appears, enter a value for the elevation and click OK. Enter a name for the new topography and click OK.
The new topography will appear in the project tree under the Topographies folder.
Once a topography has been defined, the only way to set a fixed elevation is to remove the objects that have been used to define the topography. To do this, right-click on the hyperlinked object and click Remove. You will then be able to set a fixed elevation for the topography.