Daniel: Thanks for listening ! I think your plan is a good one.
very respectfully, randy > On Apr 14, 2020, at 6:22 AM, Daniel Lee <[email protected]> wrote: > > > Hi all, > > Thanks very much for your inputs! A few points to respond to: > > From Randy: > > if the current geostationary projection parameters are deprecated, I would be > very surprised if the products for the yet to be launched GOES-R series > satellites (T and U) will reflect any changes you are proposing. > > I understand, this is an issue that's present with all of the major satellite > data producers. I do not intend to deprecate anything, but rather to provide > clarifications so that implementing things are easier. > > @steingod <https://github.com/steingod> thanks for chiming in as well, it's > very much appreciated. > > TLDR > > I myself have sought further clarification of the issue with one of our > optical imager engineers @ameraner <https://github.com/ameraner> who has been > kind enough to delve deep into the details of how sweep_angle_axis and > fixed_angle_axis are used. I think that the issue can in fact be resolved not > by adding additional attributes, but rather by clarifying in the notes how > these axes are used. PROJ probably has the best documentation on this > <https://github.com/OSGeo/PROJ/blob/master/docs/source/operations/projections/geos.rst> > - concise and understandable - and I believe that CF would profit from > similar clarity in the notes. > > I'll propose specific changes to the notes in a TBW PR, but surgery on the > Conventions needs care and so I'm summarising the specifics of the issue > below. > > Nuts and bolts > > As nicely described by PROJ > <https://github.com/OSGeo/PROJ/blob/master/docs/source/operations/projections/geos.rst> > the geostationary projection works using a gimbal model of an observation > point's viewing angle, like thus: > <https://github.com/OSGeo/PROJ/raw/master/docs/images/geos_sweep.png> > Some terms: > Sweep-angle axis: This is the outer-gimbal axis, the one whose axis is > aligned with the "base" in the figure. If you imagine this as the Earth, its > equator always describes the same great circle. > Fixed-angle axis: This is the inner-gimbal axis, the one whose axis is > attached to the "equator" of the outer-gimbal circle. Therefore its "equator" > describes a different great circle depending on the position of the outer > gimbal. > > As you can see, the definitions could be considered counterintuitive but > they're widely adopted and there's no reason to (try to) change them. > > The important thing to note is that even if you know the azimuth and > elevation of a given point, you also have to know which axis is aligned with > the conceptual outer and inner gimbal. Order of operations matters in this > case. This contradicts what I was asserting earlier and has led to > implementation difficulties, not because the attributes are incorrect, but > rather because they require some mental gymnastics to parse. > > The solution is to clarify a few things in the notes: > > How the axes correspond to a gimbal view, as this has confused developers > That the y-axis (in existing implementations, and probably for the > foreseeable future) aligns with N/S and the x-axis aligns with E/W. This is > true in practice but is not specified in our description. > — > You are receiving this because you were mentioned. > Reply to this email directly, view it on GitHub > <https://github.com/cf-convention/cf-conventions/issues/258#issuecomment-613355463>, > or unsubscribe > <https://github.com/notifications/unsubscribe-auth/AOJPXWO7RCKDKVQHQR6WHOTRMQ2PNANCNFSM4LZAULXA>. > _____________________________________ Randy C Horne ([email protected]) Principal Engineer, Excalibur Laboratories Inc. (321) 693.1074 url: http://www.excaliburlabs.com -- 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-613661554 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].
