Hi @JonathanGregory , answers inline below. > Yes I could try to moderate it. I'd certainly be a neutral moderator since > I'm quite ignorant of this! I've just read it and I have a few questions of > clarification: > > * You have a typo `thsi`.
Fixed in ee79789. > * It would be helpful to provide a diagram to show the two circles and the > two angles. The text description is pretty good, but a diagram would be > useful too. I haven't really grasped why one axis is called sweep and one > fixed, and why they aren't the other way round, since the fixed-angle axis is > the one which moves and the sweep-angle axis is stationary. I might > understand with the aid of a diagram. Excellent question - I also think that the terms are confusing, and don't know anybody who thinks that they're straightforward. Even a diagram doesn't really explain this for me; it seems that one must simply memorise it. I did consider inserting a diagram but in the whole of the CF Conventions there are only tables and code samples! That is the reason that in [the proposed text](https://github.com/cf-convention/cf-conventions/pull/259/files#diff-2d573eb68fb26a714f7ec299a6577c58R98) I link to further explanatory information and a nice diagram on the PROJ website (in fact I stole the diagram of choice and used it in [a previous comment](https://github.com/cf-convention/cf-conventions/issues/258#issuecomment-613355463)). In order to keep the document's style consistent I would prefer to use this approach, as the linked document is short and explains it as well as one can expect. It would be a small matter to introduce a diagram (or to steal the existing one) but would be a departure from the document's current style. Would you agree to leaving that part of the text as-is, sans diagram? > * Adding the Earth major axis to the perspective point height gives the > distance to the centre of the Earth only if you're above the equator, I > think. Otherwise a different radius applies. This is correct. I've struck that line in a471a2e96d712218069b472e8b8c0881fdd66e71 - as we have not limited this grid mapping to views over the equator thus far, it would potentially break backwards compatibility to do that now, and the explanation becomes clunky if we try to account for all cases. > * I don't understand `sweep_angle_axis`. Couldn't this axis be pointing in > any direction (in the rotating frame of the Earth)? How do we interpret `x` > and `y`? Could it be `z`? Why not some arbitrary direction? Yes. Taken by themselves, `x` and `y` are ambiguous, although there is no `z` here. That's why [in the proposed text](https://github.com/cf-convention/cf-conventions/pull/259/files#diff-2d573eb68fb26a714f7ec299a6577c58R83) I've narrowed it down to align `x` and `y` with the Earth's axes. This wasn't specified before but the grid mapping would never have worked otherwise because there's no way to specify them, so the assumption was already implicit. Thus I think that this does not break backwards compatibility; it simply fills a gap in the description. > * For `fixed_angle_axis`, what does "opposite" mean? Meaning if one of the axes is `x`, the other must be `y`, and vice versa. I've clarified this in c54b94e249e370a9efd1b61a29fa8bd7f3edd2c1. > Everyone is invited to comment on the proposal text in #259. Please post your > comments **here** and not in the pull request (following our GitHub > guidelines). Thanks. -- You are receiving this because you commented. Reply to this email directly or view it on GitHub: https://github.com/cf-convention/cf-conventions/issues/258#issuecomment-625721378 This list forwards relevant notifications from Github. It is distinct from [email protected], although if you do nothing, a subscription to the UCAR list will result in a subscription to this list. To unsubscribe from this list only, send a message to [email protected].
