A TopographySurface element in the Revit API represents:

• An actual topography surface which can have an arbitrary boundary and collection of points.
• A SiteSubRegion element bounded by a sketch.
• A topography surface created automatically by the introduction of a BuildingPad element.

Identify a subregion with the IsSiteSubRegion property, and access the object that provides interfaces to manipulate the subregion via AsSiteSubRegion(). Identify a topography surface associated with a building pad with the isAssociatedWithBuildingPad property, and access the associated BuildingPad element via the property AssociatedBuildingPadId. If the element does represent a subregion or a topography surface associated with a building pad, some methods of this class are inapplicable.

System Object
Autodesk.Revit.DB Element
Autodesk.Revit.DB.Architecture TopographySurface