Re: Code to produce opendap sequence

• To: Bob Simons <Bob.Simons@xxxxxxxx>
• Subject: Re: Code to produce opendap sequence
• From: John Caron <caron@xxxxxxxxxxxxxxxx>
• Date: Fri, 27 Jan 2006 15:13:59 -0700

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

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

• References:
  • Code to produce opendap sequence
    • From: Bob Simons