John Caron wrote:
> Hi Steve:
>
> CF has actually started down the path of specifying the projection
> parameters (see Appendix F), which we are taking from FGDC. So I was
> really asking for a double check on a "best practice" of how all the
> pieces should fit together, and an example added to section 5.6. We are
> currently writing code that creates CF netcdf datasets out of GRIB, so
> it seemed like a good time to double check.
Hi John,
Well, I guess my face should be a bit red that I was not even aware of Appendix
F! Obviously, I am not the right person to comment on whether you have applied
the Appendix F syntax correctly. My discomfiture is offset by the good news
that Appendix follows exactly the strategy of drawing from an existing standard
(FGDC). Great minds do think in the same gutter, I guess.
> BTW, as a philosophical matter, I would consider the specification of
> the projection as fundamental, and the specification of the explicit
> coordinate in lat, lon as redundant, because you can figure out the
> values from the projection, but not the other way around. This is not to
> say that I disagree with the decision to require the lat,lon values but
> not require the projection, however. But i would (and do) recommend
> adding the projection info. If the software knows how to use it, it
> makes visualization of the grid easier, since model grids are regular in
> the projection coordinates, and so they constitutes a kind of "natural"
> coordinate system. So I would advocate that CF recommends adding
> projection info.
I agree with your conclusion (that CF recommends adding projection info) but
qualify it by saying "where projection information is applicable". Only a
fraction of the many tri-polar, coastline following, etc. curvilinear grids in
use by modelers represent named projections. The dependence on named,
standardized projections has been a serious limitation of (say) GRIB from the
modelers' perspective. From this pov the projection name is a convenience
label that should be used to indicate special, standardized instances of
curvilinear coordinate systems.
- steve
==========================
>
>
> Steve Hankin wrote:
>
> > Hi John,
> >
> > As you know there are two basic "schools" of thought on how to
> > represent projected grids:
> >
> > 1. by naming the projection and giving its parameters (as is done
> > in GIS)
> > 2. by giving is coordinates explicitly (as is done in CF)
> >
> > What you are proposing is a mechanism to harmonize the two approaches
> > in CF files. This is an excellent idea. Since the typical purpose of
> > including this redundant (but useful) projection information is to
> > make it easier for GIS applications to access CF datasets, it makes
> > most sense to borrow the GIS encoding standards as closely as
> > possible. I presume that this is what you have done in your example
> > (??). So the question that I bounce back to you is:
> >
> > * Can you propose a particular GIS document (standard) for
> > encoding projection information and a general mapping from that
> > document into CF attributes?
> >
> > I believe this is the question that we want to address, rather than
> > the particulars of the Lambert Conformal projection.
> >
> > - steve
> >
> > ========================================================
> >
> > John Caron wrote:
> >
> >> I didnt easily find an example of using a projection coordinate systm in
> >> the CF docs, so i generated one.
> >> It would be great to get it included so i can just point people to that
> >> doc.
> >> Let me know if i did anything wrong.
> >>
> >> thanks!
> >>
> >> dimensions:
> >> y = 228;
> >> x = 306;
> >> time = 41;
> >>
> >> variables:
> >> int Lambert_Conformal;
> >> Lambert_Conformal:grid_mapping_name = "lambert_conformal_conic";
> >> Lambert_Conformal:standard_parallel = 25.0;
> >> Lambert_Conformal:longitude_of_central_meridian = 265.0;
> >> Lambert_Conformal:latitude_of_projection_origin = 25.0;
> >> double y(y);
> >> y:units = "km";
> >> y:long_name = "y coordinate of projection";
> >> y:standard_name = "projection_y_coordinate";
> >> double x(x);
> >> x:units = "km";
> >> x:long_name = "x coordinate of projection";
> >> x:standard_name = "projection_x_coordinate";
> >> double lat(y, x);
> >> lat:units = "degrees_north";
> >> lat:long_name = "latitude coordinate";
> >> lat:standard_name = "latitude";
> >> double lon(y, x);
> >> lon:units = "degrees_east";
> >> lon:long_name = "longitude coordinate";
> >> lon:standard_name = "longitude";
> >> int time(time);
> >> time:long_name = "forecast time";
> >> time:units = "hours since 2004-06-23T22:00:00Z";
> >> float Temperature(time, y, x);
> >> Temperature:units = "K";
> >> Temperature:long_name = "Temperature _at_ surface";
> >> Temperature:missing_value = 9999.0;
> >> Temperature:coordinates = "lat lon";
> >> Temperature:grid_mapping = "Lambert_Conformal";
> >>
> >> _______________________________________________
> >> CF-metadata mailing list
> >> CF-metadata at cgd.ucar.edu
> >> http://www.cgd.ucar.edu/mailman/listinfo/cf-metadata
> >>
> > --
> >
> > Steve Hankin, NOAA/PMEL -- Steven.C.Hankin at noaa.gov
> > 7600 Sand Point Way NE, Seattle, WA 98115-0070
> > ph. (206) 526-6080, FAX (206) 526-6744
> >
> >
> >------------------------------------------------------------------------
> >
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> >CF-metadata at cgd.ucar.edu
> >http://www.cgd.ucar.edu/mailman/listinfo/cf-metadata
> >
> >
>
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--
???,??,????`????,??,????`????,??,????`??????,??,???
Steve Hankin, NOAA/PMEL -- Steven.C.Hankin at noaa.gov
7600 Sand Point Way NE, Seattle, WA 98115-6349
ph. (206) 526-6080, FAX (206) 526-6744
Received on Thu Oct 14 2004 - 18:08:06 BST
