On general principles, since a MeshCoord is designed to be a "window" on other data, it should probably "have" the coord-system of the underlying coordinates from which it takes its values + metadata. ## context in UGRID At present, there is [no agreed way of attaching a coordinate-system (i.e. grid-mapping) to coordinates of a mesh](https://github.com/ugrid-conventions/ugrid-conventions/issues/64), since in CF that may only occur via a data-variable. ## context in Iris However, Iris considers the coord-system to be part of a Coord. So, it should make perfectly good sense for a coordinate of a Mesh to have one (though there is currently [an apparent bug preventing it](https://github.com/SciTools/iris/issues/5557)). Likewise, it probably makes sense for the coord-system of a MeshCoord to always follow that of the associated node coordinates, as [established for the other metadata](https://github.com/SciTools/iris/issues/4860). ## Desired implementation: Since a face- or edge-located MeshCoord takes its bounds from the (re-indexed) `node_coordinates`, and its points from the `face_coordinates` or `edge_coordinates` (which however are technically optional), it makes sense that the MeshCoord should inherit a coord_system from either of those -- and if different, that should probably be an error. For now, the coord_system of a MeshCoord is [always None, and must always be so](https://github.com/SciTools/iris/blob/v3.7.0/lib/iris/experimental/ugrid/mesh.py#L2919-L2928). ## Related * https://github.com/SciTools/iris/issues/5557 : means that the node-coords of a mesh can't actually have a coord-system, at present. * the face- or edge-coordinates, though they are technically [optional in UGRID](https://ugrid-conventions.github.io/ugrid-conventions/#2d-flexible-mesh-mixed-triangles-quadrilaterals-etc-topology), are currently [required in Iris](https://github.com/SciTools/iris/discussions/4438#bounds-no-points)