Re: Code to produce opendap sequence

Hi Bob:

Bob Simons wrote:
> Thank you for your email.
>
> John Caron wrote:
>
> > Hi Bob:
> >
> > Bob Simons wrote:
> >
> > > Ultimately, I want to use the java netcdf 2.2 library to generate an 
> > > .nc file which, when viewed with opendap, appears to have a sequence.
> > >
> > > My reading of the netcdf java library leads me to believe I need to 
> > > create a Structure. Does anyone have sample code showing how to 
> > > create a  .nc file with a Structure using the java netcdf library?
> >
> >
> >
> > Netcdf-3 files are limited in what they can represent; Netcdf-4, when 
> > available will be able to represent the entire CDM.
> >
> > The only possible structure in a Netcdf-3 file is by using the 
> > unlimited dimension. See this doc for details:
> >
> > http://www.unidata.ucar.edu/software/netcdf-java/formats/RecordsInNetcdf3.html 
>
>
>
> There are no programming details there. That document is filled with 
> examples of the what the NCDUMP of the resulting file is, but there is 
> no Java code showing how to make the files.  In other words, it says 
> what can be done, not how to do it.  I ask again: can you point me to 
> sample Java code to write files with structures?

Really, netcdf-3 files dont support structures. Nj22 will recognize the "record" structure, which you get by using unlimited dimensions. The example in the nj22 manual appendix "Create a netCDF file" shows creating a file with unlimited dimension. Its a bit out of date, im afraid to say, you need to use 

 Dimension timeDim = ncfile.addDimension("time", -1, true, true, false);

instead of

 Dimension timeDim = ncfile.addDimension("time", -1);
        

But this wont be useful within an arbitrary opendap server/client. So if we 
rewind a few steps, I guess it would help me to know what opendap server you 
want to use, and perhaps a bit more of the context.



> > All the variables that have the unlimited dimension get put into the 
> > "record" structure, and if you call netcdfFile.addRecordStructure(), a 
> > Structure is added. The TDS-OpenDAP server, for example, will serve 
> > this as an Array of Structures. Since the length of it is known, 
> > theres no reason to make it into a Sequence. Currently theres no way 
> > to get nested Sequences.
> >
> > So the "when viewed with opendap" part depends on what opendap server 
> > you use.
> >
> > A Sequence is special because you are promising to allow relational 
> > constraint expressions on the fields. So your server has to deal with 
> > that, which is non-trivial. We are still considering if its possible 
> > to support sequences in the CDM.
> >
> > > I have experimented with netcdfFile.addRecordStructure(), but can't 
> > > figure out how to access the Structure in order to call 
> > > addMemberVariable in order to add the variables to it.
> >
> >
> >
> > Note that you can create an in-memory NetcdfFile with 
> > addMemberVariable() etc, but you cannot save this to a netcdf-3 file. 
> > As I said, netcdf-4 will allow other things, including (I think) 
> > variable length nested Structures, which are Sequences, but the 
> > question of supporting relational constraint expressions remains open.
> >
> > John
>
> I don't think you answered my other email. Here are the questions in 
> contained:
>
>     Can anyone point me to a working example which uses
>     PointObsDataset, StationObsDataset and/or TrajectoryObsDataset?
>
>     Is there any documentation for these classes (other than
>     the JavaDocs)?

Im sorry theres not at this time. Those are experimental APIs that we are 
working on currently. The ToolsUI code uses them, but in a rather complex way. 
If you want, look at the ucar.nc2.ui.PointObsViewer class.





> Thank you.
>
> Sincerely,
>
> Bob Simons
> Satellite Data Product Manager
> Environmental Research Division
> NOAA Southwest Fisheries Science Center
> 1352 Lighthouse Ave
> Pacific Grove, CA 93950-2079
> (831)658-3205
> address@hidden
> <>< <>< <>< <>< <>< <>< <>< <>< <><