[Dune] Non-unique coordinate mappings of Codim(1) entities?

[Christian Engwer]

Hi Christian,

> I am trying to implement  a hybrid DG method using DUNE and I have a
> problem with the mapping feature of the intersections. In addition
> to my element DOFs, I have an additional function living on the
> trace of the grid, i.e. I have some DOFs distributed on the faces of
> the mesh. The intersections of the elements seem to be associated
> with the codim 1 entities of the mesh, but the orientation of their
> geometries seems to be not unique in my experiments (using
> ALUSimplexGrid<2,2>).

no, the intersections do not (necessarily) correspond to the codim1
entities. Intersections are of codim1, but they are not an entity of
the grid. 

Think about hanging node: 

  +-------+---+
  |       |   |
  |       +---+
  |       |   |
  +-------+---+

on the left you have the big cell with one big face intersecting with
two neighboring cells. On the right you have 2 small faces
intersecting with the big cell. The right face of the big cell is
divide into two intersections, which correspond (geometrically) to the
left face of the two small cells.

Cheers
Christian