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Re: Reference time in aggregated files

Hi Roy:

There is a problem when the units of the aggregation coordinate change from 
file to file.

There is a way around this, by generating the needed coordinate values and 
placing in the NcML, eg:

<netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2";>

 <variable name="time">
    <attribute name="units" value="months since 2000-01-01 00:00"/>
    <values>0, 1, 2</values>

 <aggregation dimName="time" type="joinNew">
   <variableAgg name="T"/>
   <netcdf location="file:test/data/ncml/nc/time0.nc" />
   <netcdf location="file:test/data/ncml/nc/time1.nc" />
   <netcdf location="file:test/data/ncml/nc/time2.nc" />


I think this also works for joinExisting, in that it would override the 
existing values inside the file(s) (I need to double check that). I dont think 
there will be a performance penalty.

If the values are regularly spaced, you can use:
 <values start="0" increment = "1"/>

I think that this case of regular spacing may work for scanned datasets, in 
that the number of points will be taken from the files scanned (again i need to 
double check that).

Roy Mendelssohn wrote:

This question has arisen working with a partner - and while I am pretty certain I know the answer to this question, I want to make sure what is the correct answer.

The netcdf files that these people are creating have a changing reference time fin the files. So, for example, say they are creating monthly files, then the 12 files for any year all have the same base reference data (the start of that year), and this changes with each year.

My guess is that this will most likely break the aggregation - and if not at least slow it down considerably. Is my guess correct?


-Roy M.

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