Well, I've realised that this is a lot simpler than I thought. There is, I believe now, no problem with conventions - they don't disallow anything you might want to do, nor allow anything that you shouldn't do.
All that is needed, I think, is to amend the [conformance document](http://cfconventions.org/Data/cf-documents/requirements-recommendations/requirements-recommendations-1.7.html) to allow for the fact that compression by gathering may result in some indirection when finding the "real" dimensions of auxiliary coordinate variables. The existing text reads: _The dimensions of each auxiliary coordinate must be a subset of the dimensions of the variable they are attached to, with two exceptions. First, a label variable which will have a trailing dimension for the maximum string length. Second a ragged array (Chapter 9, Discrete sampling geometries and Appendix H) uses special, more indirect, methods to connect the data and coordinates._ I propose adding a third exception (new text in **bold**): The dimensions of each auxiliary coordinate must be a subset of the dimensions of the variable they are attached to, with **three** exceptions. First, a label variable which will have a trailing dimension for the maximum string length. Second, a ragged array (Chapter 9, Discrete sampling geometries and Appendix H) uses special, more indirect, methods to connect the data and coordinates. **Third, a data variable that has been compressed by gathering (chapter 8.2. Compression by Gathering) may also use special, more indirect, methods to connect the data and coordinates.** -- You are receiving this because you are subscribed to this thread. Reply to this email directly or view it on GitHub: https://github.com/cf-convention/cf-conventions/issues/147#issuecomment-429275471
