Re: [netcdfgroup] defining dimensions in groups

• To: "Pedro Vicente" <pedro.vicente@xxxxxxxxxxxxxxxxxx>
• Subject: Re: [netcdfgroup] defining dimensions in groups
• From: Russ Rew <russ@xxxxxxxxxxxxxxxx>
• Date: Fri, 18 Jan 2013 14:50:13 -0700

Hi Pedro,

> I have a couple questions regarding the definition of dimensions in
> groups;
> 
> Consider the following CDL definition
> 
> netcdf in_grp {
> dimensions:
>  lon=3D3;
> variables:
>  //coordinate variable
>  float lon(lon);
> data:
>  lon=3D1,2,3;
> group: g1 {=20
>  dimensions:
>  lon=3D2;
>  variables:
>  float lon(lon);
>  float data(lon)
>  data:
>  lon=3D1,2;=20
>  } // end g1
> } // end
> 
> There is a "lon" dimension/coordinate variable both at root and in group
> /g1 and they have different dimensions, so variable named "data" has 2
> dimensions with the same name in scope,
> and somehow has to choose one
> 
> Just wanted to confirm
> 
> 1) Is this definition legal?

Yes, except that the line 

  float data(lon)

needs to be terminated with a ';'.

> 2) What is the algorithm that the netCDF library uses to distinguish
> between both dimensions in scope for the variable /g1/data?

The innermost dimension is used, so 

  lon=2;

is the dimension of the 'data' variable.

> 3) Defining "lon" as a coordinate variable instead of just a dimension
> name, does this definition has any consequence for the way this is
> handled by the library for this case?

No, I don't think so.  NetCDF shared dimensions and inherited dimensions
aren't directly representable in the HDF5 data model, so there are a few
artifacts created in the HDF5 representation to make things mostly work
by using HDF5 dimension scales and some hidden attributes.  More details
are in John Caron's blog:

  http://www.unidata.ucar.edu/blogs/developer/en/entry/dimensions_scales

> Can someone from Unidata please point me to the location in the source
> code where this is handled?

The ncgen utility parses the CDL and selects the innermost dimension for
each variable.  You can use the ncgen and ncdump utilities 

   ncgen -b in_grp.cdl; ncdump in_grp.nc

to verify that data has two values

   data = _, _ ;

which in this case are _FillValue for floats, denoted as '_'.

The source code for ncgen is mostly in the yacc (or bison) grammar
ncgen/ncgen.y.

--Russ

• References:
  • [netcdfgroup] defining dimensions in groups
    • From: Pedro Vicente