Get the PlanTopology of a given level in a given phase. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
Syntax
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Parameters
- level
 -  Type:  Autodesk.Revit.DB Level  
The level of the Plan Topology. 
- phase
 -  Type:  Autodesk.Revit.DB Phase  
The phase of the Plan Topology. 
Remarks
 Accessing plan topology requires that document is modifiable as it will actually trigger calculating plan topology if they have not been calculated yet. The time necessary for the calculations may be significant and should be considered.