Problem aggregating along time dimension

• To: netcdf-java <netcdf-java@xxxxxxxxxxxxxxxx>
• Subject: Problem aggregating along time dimension
• From: "Jon Blower" <jdb@xxxxxxxxxxxxxxxxxxxx>
• Date: Fri, 9 Feb 2007 13:32:43 +0000

Dear all,

I've come across a problem when aggregating a dataset across the time
dimension using NcML and the latest (2.2.18) nj22 library.  I would
like to aggregate a number of files, which represent timesteps of an
ocean forecast model (one timestep per file).  I am aggregating using
an NcML file like this:

<netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2";>
  <aggregation dimName="t" type="joinExisting">
    <scan location="/path/to/files" suffix=".nc"/>
  </aggregation>
</netcdf>

When reading this NcML file using NetcdfDataset.openDataset() I get an
aggregated dataset with the correct number of timesteps BUT all the
values along the time axis are the same.

After much head-scratching I think I've found the source of the
problem, but I don't have a solution.  In each individual file, the
time dimension is specified with different units but the values remain
the same:

File 1: time units: seconds since 2007-02-06 00:00:00, value: 43200
File 2: time units: seconds since 2007-02-07 00:00:00, value: 43200

This seems to confuse the aggregation code, which I assume expects the
units to remain constant between files and the values to change.

Is there a solution or workaround for this?  I don't control how the
netCDF files are made so I don't think I can change them.  Any help
will be much appreciated.

Regards, Jon

--
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Dr Jon Blower              Tel: +44 118 378 5213 (direct line)
Technical Director         Tel: +44 118 378 8741 (ESSC)
Reading e-Science Centre   Fax: +44 118 378 6413
ESSC                       Email: jdb@xxxxxxxxxxxxxxxxxxxx
University of Reading
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Reading RG6 6AL, UK
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    • From: John Caron