Hello Iain,
Thank you for your response.
It looks like I might need to work on this as well. Perhaps we can work
on it together. But we are still investigating whether we really need it
or not, so I will let you know.
I would like to know what you found out in your investigation stage
about the problem. I would expect that changes need to be made to
netcdf-java to support it as well?
Kind Regards
Niels
On 06-10-16 21:57, Sean Arms wrote:
Hi Niles,
In general, netCDF-Java can handle 2D time coordinates (runtime, time).
CF conventions, which you are using due to the conventions attribute
you added, explains the case of 2D lat/lon variables, but as far as I
can tell, not a two dimensional time coordinate variable. Following
CF, a 2D lat/lon variable is recognized as a coordinate variable if
another variable lists it in its "coordinate" attribute. See
http://cfconventions.org/Data/cf-conventions/cf-conventions-1.6/build/cf-conventions.html#idp5559280
for an example. Since a 2D time coordinate was not explicitly
mentioned, netCDF-Java appears to have chosen to use the same
approach, that is, somewhere in the dataset, the 2D time coordinate
must be listed in someones "coordinate" attribute to be picked up as a
coordinate variable. For your case, it is not listed explicitly in a
coordinate attribute, so it's not being detected as a coordinate variable.
If you want to aggregate using a 2D time (that is, keep time as an
aggregation variable), then you need to add the appropriate coordinate
attribute to each aggregation variable that will use the 2D time. So,
for example,
water_u, salinity, water_v, and water_temp will need the attribute
"coordinates: runtime time depth lat lon", and surf_el will need the
attribute "coordinates: runtime time lat lon"
Circling back to Ben's question which made me feel a bit uneasy, he
asked "Sean, do you expect aggregations to behave the same as single
files with the same Conventions? It looks like Niels has identified a
difference." to which I responded yes, I expect them to behave the
same. However, what was happening here is that time was being upped to
a 2D coordinate in the aggregation, which then added a new condition
for it to be recognized as a coordinate variable - that is, was it
explicitly listed in someones coordinate attribute? In this case, it
wasn't explicitly listed in a coordinate attribute, so it was
interpreted as a non-coordinate variable in the aggregated dataset.
Now, one could argue that maybe netCDF-Java should automagically
recognize that time was a 1D coordinate before it was aggregated to
2D, and thus should automatically be treated as a 2D coordinate
variable without being listed in another variables coordinate
attribute, but I think it's better to force things to be explicit and
allow the flexibility for this not to happen. Also, I am not sure the
aggregation code tracks things that closely, so automatically doing
this would likely slow down aggregations even further. Yuck.
Cheers,
Sean
On Wed, Oct 5, 2016 at 3:35 AM, Niels Charlier <niels@xxxxxxxxx
<mailto:niels@xxxxxxxxx>> wrote:
Sean, Ben,
Isn't issue #2 related to the lack of support for two-dimension
coordinate variables that the client talked about with regards to
the runtime aggregations?
Sorry if I am stating the obvious here, I have only been learning
while working on this.
I did not fully comprehended the relationship between the two
issues at first. But reading back my description of what goes
wrong in the code, it all starts with the time variable not being
recognised as an axis because of the runtime dimension being added
to it as a second dimension.
Still, the aggregation works fine if I leave out the time
variable. I am still not fully understanding how its presence as
"variableAgg" affects the end result. I don't see any examples
online where axis variables are included as variableAgg. This
isn't exactly clear to me. Please enlighten me on that point.
Kind Regards
Niels
On 09/08/2016 02:51 PM, Niels Charlier wrote:
2. removal of the "time" variable in the aggregation (see
commented line in attached .ncml file)
This issue is related to the previous one, it is again about
/makeCoordinateSystemsImplicit/ versus
/makeCoordinateSystemsMaximal/.
* If the "time" variable is included as aggregation variable,
it also gets the "runtime" dimension added to its dimensions.
* As a consequence, netcdf-java does not recognise it as an AXIS.
* As a consequence, /makeCoordinateSystemsImplicit /fails to
include it in the CRS's for the actual variables (water_u,
salinity, etc...).
* As a consequence, /makeCoordinateSystemsMaximal /creates a
CRS for these variables. However, it puts the dimensions in the
order of the /dimension variable/ list
(rather than the variable's own dimensions), which is
completely arbitrary as far as I can see.
* "runtime" ends up as the last axis instead of the first.
This is inconsistent with the order of dimensions in the
variable. Geotools fails on this inconsistency.
I am not sure whether the fault here lies with the .ncml
file, geotools or netcdf-java.
