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[CF-metadata] Conventions for Unstructured Grid Data

From: Rich Signell <rsignell>
Date: Mon, 15 Nov 2004 17:35:41 -0500

Jonathan,

I just had a long talk with Tom Gross, and we have the following proposal
for handling unstructured grids:

(1) Add the following standard_names to CF:

Horizontal_Triangular_Element_Incidence_List
Boundary_Segment_Node_List

(2) remove units attributes from the variables that correspond to
these names.

(3) Define the upcoming CF Convention "parameters" attribute
as "index_start_0" or "index_start_1"

The resulting unstructured CDL would thus look like:

netcdf umesh_cf {
dimensions:
        node = 9700 ;
        nele = 17925 ;
        nbnd = 1476 ;
        nface = 3 ;
        nbi = 4 ;
        sigma = 1 ;
        time = UNLIMITED ; // (0 currently)
variables:
        int ele(nele, nface) ;
                ele:standard_name =
"Horizontal_Triangular_Element_Incidence_List" ;
                ele:parameters = "index_start_1" ;
        int bnd(nbnd, nbi) ;
                bnd:standard_name = "Boundary_Segment_Node_List" ;
                bnd:parameters = "index_start_1" ;
        float time(time) ;
                time:long_name = "Time" ;
                time:units = "days since 2003-01-01 0:00:00 00:00" ;
                time:base_date = 2003, 1, 1, 0 ;
                time:standard_name = "time" ;
        float lon(node) ;
                lon:long_name = "Longitude" ;
                lon:units = "degrees_east" ;
                lon:standard_name = "longitude" ;
        float lat(node) ;
                lat:long_name = "Latitude" ;
                lat:units = "degrees_north" ;
                lat:standard_name = "latitude" ;
        float depth(node) ;
                depth:long_name = "Bathymetry" ;
                depth:units = "meters" ;
                depth:positive = "down" ;
                depth:standard_name = "depth" ;
                                                                                                                

// global attributes:
                :file_type = "FEM" ;
                :Conventions = "CF-1.0" ;
                :grid_type = "Triangular" ;
                :z_type = "sigma" ;
                :model = "quoddyv511" ;
                :title = "Chesapeake Bay FEM" ;
                :comment = "zzrtquoddylinux Twice Daily Runs" ;
                :source = "bassmmap" ;
                :institution = "NOAA/NOS/OCS/CSDL/MMAP" ;
                :history = "original" ;
                :references = "tom.gross at noaa.gov" ;
                :creation_date = "2004-05-06 9:18:23 04:00" ;
}

Jonathan Gregory wrote:

>Dear Tom
>
>I think we didn't finish the earlier discussion, back in February.
>
>We need to have standard names for the various quantities that define your
>mesh, such as your "Horizontal Triangular Element Incidence List" and "Boundary
>Segment Node List". We agreed that the convention for whether it numbers from
>0 or 1 needs to be stated. You've suggested doing this with the units
>attribute e.g. units="index_start_1". I think that it would be better to put
>the "start 0" or "start 1" information into the standard name itself because
>
>(a) units="index_start_1" or similar is not a legal udunit. Hence it would
>cause a problem to anything which uses udunits to interpret the attribute.
>
>(b) we do not use units to indicate sign conventions, and this strikes me as
>a similar kind of issue.
>
>(c) it is absolutely essential that this information is specified. With
>physical quantities you could often make an intelligent guess if the units
>weren't specified, but in this case a guess would be hard to make. Since it is
>essential, putting it in the standard name is good because it's unavoidable.
>
>However a compromise might be to consider putting this into a standard_name_
>parameter which has to be specified as well as the standard name, such as we
>have been discussing in another thread.
>
>Any comments?
>
>Best wishes
>
>Jonathan
>_______________________________________________
>CF-metadata mailing list
>CF-metadata at cgd.ucar.edu
>http://www.cgd.ucar.edu/mailman/listinfo/cf-metadata
>
>


-- 
Richard P. Signell           rsignell at usgs.gov
U.S. Geological Survey       Phone: (508) 457-2229
384 Woods Hole Road          Fax:   (508) 457-2310
Woods Hole, MA  02543-1598  
Received on Mon Nov 15 2004 - 15:35:41 GMT

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