diff --git a/Modules/IGT/IGTFilters/mitkTrackingVolumeGenerator.cpp b/Modules/IGT/IGTFilters/mitkTrackingVolumeGenerator.cpp
index 260189b175..1c53090768 100644
--- a/Modules/IGT/IGTFilters/mitkTrackingVolumeGenerator.cpp
+++ b/Modules/IGT/IGTFilters/mitkTrackingVolumeGenerator.cpp
@@ -1,106 +1,106 @@
 /*=========================================================================
 
 Program:   Medical Imaging & Interaction Toolkit
 Language:  C++
 Date:      $Date: 2011-01-18 13:22:38 +0100 (Di, 18 Jan 2011) $
 Version:   $Revision: 28959 $
 
 Copyright (c) German Cancer Research Center, Division of Medical and
 Biological Informatics. All rights reserved.
 See MITKCopyright.txt or http://www.mitk.org/copyright.html for details.
 
 This software is distributed WITHOUT ANY WARRANTY; without even
 the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
 PURPOSE.  See the above copyright notices for more information.
 
 =========================================================================*/
 
 #include "mitkTrackingVolumeGenerator.h"
 #include "mitkSTLFileReader.h"
 #include "mitkStandardFileLocations.h"
 #include "mitkConfig.h"
 #include <vtkCubeSource.h>
 #include <mitkTrackingTypes.h>
 #include <mitkTrackingDevice.h>
 #include <mitkVirtualTrackingDevice.h>
 #include <vtkSmartPointer.h>
 #include <mitkSurface.h>
 
 mitk::TrackingVolumeGenerator::TrackingVolumeGenerator()
 {
     std::string volumeDir = MITK_ROOT;
     volumeDir += "Modules/IGT/IGTTrackingDevices/TrackingVolumeData"; //folder which contains the trackingdevices configs
     mitk::StandardFileLocations::GetInstance()->AddDirectoryForSearch( volumeDir.c_str(), false ); //add this directory to StdFileLocations for the search
 
     m_TrackingDeviceType = mitk::TrackingSystemNotSpecified;
 }
 
 
 void mitk::TrackingVolumeGenerator::SetTrackingDevice (mitk::TrackingDevice::Pointer tracker)
 {
     this->m_TrackingDeviceType = tracker->GetType();
 }
 
 void mitk::TrackingVolumeGenerator::GenerateData()
 {
     mitk::Surface::Pointer output = this->GetOutput();//the surface wich represents the tracking volume
 
     std::string filename = "";
 
     switch(m_TrackingDeviceType)
     {
     case mitk::ClaronMicron:
         filename = mitk::StandardFileLocations::GetInstance()->FindFile("ClaronMicron.stl");
         break;
     case mitk::IntuitiveDaVinci:
         filename = mitk::StandardFileLocations::GetInstance()->FindFile("IntuitiveDaVinci.stl");
         break;
     case mitk::NDIPolaris:
         filename = mitk::StandardFileLocations::GetInstance()->FindFile("NDIPolaris.stl");
         break;
     case mitk::NDIAurora:
         filename = mitk::StandardFileLocations::GetInstance()->FindFile("NDIAurora.stl");
         break;
     case mitk::TrackingSystemNotSpecified:
     case mitk::VirtualTracker:
         {
            vtkSmartPointer<vtkCubeSource> cubeSource = vtkSmartPointer<vtkCubeSource>::New();
             double bounds[6];
             bounds[0] = bounds[2] = bounds[4] = -400.0;  // initialize bounds to -400 ... +400 cube. This is the default value of the
             bounds[1] = bounds[3] = bounds[5] =  400.0;  // virtual tracking device, but it can be changed. In that case,
             // the tracking volume polydata has be updated manually
             cubeSource->SetBounds(bounds);
             cubeSource->GetOutput()->Update();
 
             output->SetVtkPolyData(cubeSource->GetOutput()); //set the vtkCubeSource as polyData of the surface
             return;
         }
     default:
         {
-            output->SetVtkPolyData(vtkPolyData::New());
+            output->SetVtkPolyData(vtkPolyData::New()); //initialize with empty poly data (otherwise old surfaces may be returned) => so an empty surface is returned
             MITK_ERROR<< "No STL to given TrackingDevice found";
             return;
         }
 
     }
 
     if (filename.empty())
     {
         MITK_ERROR << "Filename is empty";
         return;
     }
 
     mitk::STLFileReader::Pointer stlReader = mitk::STLFileReader::New();
     stlReader->SetFileName( filename.c_str() );
     stlReader->Update();
 
     if ( stlReader->GetOutput() == NULL)
     {
         MITK_ERROR << "Error while reading file";
         return ;
     }
     output->SetVtkPolyData( stlReader->GetOutput()->GetVtkPolyData());//set the visible trackingvolume
 }