Re: More Songs about Coordinate Systems and Buildings

William Weibel (weibel@uniblab.atmos.Ucla.EDU)
Thu, 7 Aug 1997 16:10:31 -0700 (PDT) 

Just some comments from an atmospheric science worker on a couple of items.

I wanted to contribute to the ideas regarding syntax for referential
attributes.  But the discussion has gotten a bit too detailed for me
to follow.

John Caron's definition of a "coordinate system" seems nearly general
enough.  I would save terms such as "physical space" and "location"
for the definitions of "spatial coordinate system" and the like.
Unfortunately, I can't think of any more general alternatives at the
moment.

> 
> > 
> > > ...
> > > ----------------------------------------------------------------------------
> > >
> > > Examples
> > >
> > > Shorthand "solutions" of the motivating examples :
> > 
> > I'll combine parts of your mail and offer an alternative set of
> > solutions here.  The two approaches are so close that I think little
> > additional explanation is required.  Basically, my approach differs
> > in the following ways:

> > ...
> >    a) define global "coordinate maps", a la Gary Granger
> > 
> >    b) the "coordinate map" name should have a prefix indicating the
> >         basic coordinate system (cartesian, cylindrical, spherical,
> >         geodetic, ...)

One thing to keep in mind is a distinction between the specifying
enough degrees of freedom to describe a field of data, and how some
application may display the data.  For nonspatial coordinates, such
as frequency, magnetic field strength, or happiness, there is no
inherent coordinate map.


> > >
> > >   7. vector valued variables, for example:
> > >       vector(lev, 3)
> > >       velocity(lat,lon,component)
> > >           component(3) = "u", "v", "w"
> > 
> >   Your solution:
> > >           :coordinates = "lev";
> > >           :coordinates = "lat lon components";
> >   My solution:
> >       This is not a "coordinates" issue.  "Associating" separate scalar
> >       quantities into a vector quantity is a useful goal, but I haven't
> >       had time to give it enough thought.
> 
> The u,v,w example is tricky because there is another level of
> information we want to associate here, namely the spatial direction of
> each of those components. All I intended here was to label the three
> numbers. 

I think this is a case where the distinction between inherent degrees
of freedom and display issues (or analysis issues) comes up again.

> There is some interesting problem here on specifying a wind vector,
> having both a location and a direction. Anyone want to offer an axample
> and solution? 

I wonder if an attempt is being made to specify too much too soon.

Typically, we declare each component of a vector field as a scalar variable.
When we look at the data (in this case, using GrADS) we say

	display u;v

so the user explicitly declares at display time that u is the x component
and v is the y component (because cartesian coordinates are assumed).

> > >  11. multiple time coordinates:
> > >
> > >       var(time)
> > >           year(time)
> > >           day_of_year(time)
> > >           second_of_day(time)
> > 
> >   Your solution:
> > >           :coordinates = "year, day_of_year, second_of_day";
> > 
> >   My solution:
> >     short time(time);  // simply index values, unless you
> >                        //  have a traditional value, too
> >           time:component="year, day_of_year, second_of_day";  // a la GDT
> 
> I admit I havent dealt with this example.
> 
> In the GDT solution I dislike putting indices in the time(time)
> variable; The semantics of "component" just gives you a 3-tuple so you
> lose the fact that year.day.second is ordered. 
> 

I'm afraid others are much better qualified than me to comment on time.
But the expression "year.day.second" looks a little scary.
I doubt whether astronomers would adopt such a thing.
Are these ideas compatible with the COARDS convention
(http://ferret.wrc.noaa.gov/noaa_coop/coop_cdf_profile.html)?


Sincerely,
William Weibel

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   William Weibel                           weibel@atmos.ucla.edu
   UCLA Department of Atmospheric Sciences  Tel. (310)206-4441       \\\\/
   Los Angeles, CA  90095-1565              Fax  (310)206-5219        O-O
   U.S.A.                                                              |
                                                                       - 
  ... to know even one life has breathed easier because you have lived; 
  this is to have succeeded.			--Ralph Waldo Emerson
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