Topographic elements are not visible in your current view. You can modify View Range settings in the view's Properties or Visibility settings in Visibility/Graphics.
Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 25.3.0.0 (25.3.0.0)
Syntax
C#
public static FailureDefinitionId SiteInvisible { get; }