[help wanted] understand the optional_attributes

[xldeltares]

In hydromt-delft3dfm, when creating Ugrid1d and converts it back to a Ugrid dataset to_dataset(), we encountered the use of optional_attributes. By default, optional_attributes is set to False. in that case, network1d_edge_x and network1d_edge_y are not written into the dataset. (But if the Ugrid1d is parsed from a file, then the network1d_edge_x and network1d_edge_y seem to become not optional anymore).

The question is: is that indeed the desired behavior? for our case, we always needed these optional_attributes therefore we needed to set it to True explicitly wherever applicable. Therefore, we figure optional_attributes as True would be more suitable. But maybe there are cases where this argument should be False? Could more info be provided please?

[Huite]

Hi @xldeltares,

As the name implies, the optional attributes are optional. They are not required to define the topology unambiguously: e.g. if you have a Ugrid1d topology, all that is needed is the node_x, the node_y, and the edge_node_connectivity. All other attributes can be derived from these values. Similarly for Ugrid2d and node_x, node_y, and the face_node_connectivity.

However, the UGRID conventions also describe "Optionally required attributes": http://ugrid-conventions.github.io/ugrid-conventions/

I think the best example is for the edge_node_connectivity for a Ugrid2d topology. Let's say we have a grid with a single face:

3 -- 2
|    |
0 -- 1

face_node_connectivity = [[0, 1, 2, 3]]

This contains four edges which are unique:

edge_node_connectivity = [
    [0 , 1],
    [1,  2],
    [2,  3],
    [3,  0],
]

However, the order isn't prescribed, this is valid too:

edge_node_connectivity = [
    [3,  0],
    [1,  2],
    [2,  3],
    [0 , 1],
]

Now let's say you have a value on every edge (e.g. the amount of water flowing). To interpret those correctly, you need to know how the edges are ordered... you need to know the associated edge_node_connectivity.

I've added the optional_attributes flag specifically because DELWAQ requires edge_coordinates and even edge_face_connectivity, and DELWAQ apparently doesn't derive them itself.

Anyway, the basic logic is as follows:

• xugrid initialises topologies with the minimum amount of attributes: only the only that are strictly required.
• if a grid topology is initialized with optionally required attributes, it will also preserve them. (This is e.g. required to keep us from messing up data defined on the edges of a 2d grid as shown above.)
• If optional_attributes=True, it will write ALL attributes as defined here: http://ugrid-conventions.github.io/ugrid-conventions/

Of course I may have made some mistakes where it doesn't quite behave like this. Different software also has different expectations (e.g. DELWAQ requires all these optional attributes, but other software might only accept the required attributes...).

[Huite]

Hi @xldeltares,

I'm closing this for now. If you have more question, feel free to re-open.