Hi there,
apologies for this, i am sure its a stupid question but i have spent too 
much time already. I have a large number of netcdf files that are 
actually arranged as a grid/tiled fashion, (file-001-001, 
file-001-002..) .. representing the same variables and dimensions and 
points in time, just different spatially.
i want to make a virtual dataset that is the combination of all the 
tiles..which i will then map via WMS.
i have tried aggregating on the lat/long dimensions like this:
<netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2">
<aggregation dimName="latitude" type="joinExisting" recheckEvery="4 sec">
<netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2">
<aggregation dimName="longitude" type="joinExisting" recheckEvery="4 sec">
<scan location="E:/save/agg/" regExp="files_26_04_2011.nc.*" />
</aggregation>
</netcdf>
</aggregation>
</netcdf>
 but it is not working, i get a 'smeared' version of a single tile.. 
and tried doing a simple union like this:
<netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2">
<aggregation type="union">
<scan location="E:/save/agg/" regExp="files_26_04_2011.nc.*" />
</aggregation>
</netcdf>
but i get only one tile rendered via WMS.
I cannot find any references to aggregations based on any dimension 
other than time, and am wondering if this is even possible.. but it 
seems like such a common thing to do i must be doing something dumb. i 
found one reference to a type of aggregation called 'tiled' but no doco 
anywhere for it.. maybe this is what i need ?
any suggestions would be appreciated
cheers
-i