But TopoGridFilter outputs vtkCellArray and vtkPoints - those are not “arrays” that can be found by vtkGradientFilter?
And how do I get the vtkGradientFilter output connected to the mapper? I currently do the following:
mapper->SetInputConnection(gradientFilter_->GetOutputPort());
But the visualized data looks identical to the vtkElevationFilter output.
How do I examine the gradientFilter output? (Man, that sounds familiar, right? 
) I do this:
vtkPolyData *polyData = gradientFilter->GetPolyDataOutput();
vtkCellArray *cells = polyData->GetPolys();
vtkPoints *points = polyData->GetPoints();
But cells and points look identical to the filter input…
Thanks!