[netcdf-java] joinExisting aggregation

Hello,

I'm trying to aggregate a set on Netcdf files and have run into a problem. The dimension I am trying to aggregate over is called datum, but there is no datum variable, just a number of variables with dimension datum. So I get an error like:

no coordinate variable for agg dimension= datum


My understanding is that I can create a variable datum using NCML in this way:

<?xml version="1.0" encoding="UTF-8"?>
<netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2";>
         <variable name="datum" shape="datum" type="int">
            <values start="0" increment="1" />
        </variable>
<aggregation dimName="datum" type="joinExisting" recheckEvery="15 min">
               <scan location="." suffix=".nc" />
         </aggregation>
 </netcdf>

But I continue to get the same error, as if no new variable is created. Perhaps I am unable to use this method with the scan object? Or must the dimension and variable be called "time".

Any insight would be appreciated.

Thanks,
Eli