ExtractSurface extracts more than I would like


I just started using OpenGeoSys with the intention of extracting the uppermost surface of an unstructured grid in .vtu format. I used the ExtractSurface tool with the following parameters:

-i MyMesh.vtu -o TopSurface.vtu

The following picture shows myMesh (top left), the TopSurface output (top right), and a temporary workaround with a ParaView threshold filter (bottom):

I suppose the multiple ‘top’ slices might be theoretically correct because this is unstructured mesh in format. However, in practice the mesh is defined in layer/row/column format, there is a very clear ‘top’ to the viewer. Is there a way extract the actual top surface only? Note: this is just a simple example mesh for testing, my real meshes will be more interesting!

I seem to be able to get a temporary work-around in ParaView with a threshold filter, but ideally I’m lookign to access the global top elements of the mesh, rather than the local top elements.