Id of level which represents ground level.
Namespace: Autodesk.Revit.DB.Analysis
Assembly: RevitAPI (in RevitAPI.dll) Version: 25.0.0.0 (25.0.0.0)
public ElementId GroundPlane { get; set; }
Exception
Condition
When setting this property: The input element is not a level or invalidElementId.
When setting this property: A non-optional argument was null
The ground plane defines what is above and below ground for Conceptual Energy Analysis. The ground plane can optionally used for the rendering cast shadows.