Stephan,
You are exactly right that the only way provided by CF to indicate that
a group of coordinate variables (both 'true' and auxiliary) and
coordinate reference system (CRS) spec compose a set (or whatever name
you wish to call it) that should be treated as a unit is to to have them
all referenced by a data variable. I think this isn't such a great idea,
but it is what it is.
One way to do this within CF is to create a scalar variable that lists
all of the coordinates of both types in a coordinates attribute and
identifies the CRS in a grid_mapping attribute. The variable serves only
to indicate that these all compose a set.
Grace and peace,
Jim
On 10/14/14, 1:07 PM, Seth McGinnis wrote:
Hi Stephan,
I think you'll run into trouble with surface variables that have
different reference heights -- like 10-meter winds vs 2-meter air
temperature or 500-mb height. Those will have the same dimensionality
but different auxiliary coordinates. (Especially if you specify height
using a scalar coordinate, in which case they'll also have the same
dimensions as a purely 2-D surface variable like precipitation or skin
temperature.)
Cheers,
--Seth
On 10/13/14 12:30 PM, Stephan Hoyer wrote:
Hi Seth,
So in my Python library for working with netCDF-like data [1],
coordinates are a property of the dataset, rather than being associated
with particular data variables. Within this context, it is possible to
have a dataset that only contains coordinates. I agree that such a
dataset is not terribly useful on its own (although we have indeed used
them as templates in the past), but I like to cover all the edge cases
in my code :).
I think it would be reasonable for CF conventions to handle the
possibility of auxiliary coordinates associated with an entire dataset.
It's straightforward to implicitly associate auxiliary coordinates with
data variables based on shared dimensions. I can't think of any case
where I would not want to automatically associate all dimension
compatible auxiliary coordinates to a data variable, but I may be
missing something.
In truth, I suppose it doesn't matter that much if other libraries
reading the file consider lat & lon to be coordinates or not. I just
want to be able to ensure that my own library can faithfully
serialize/deserialize these datasets, and ideally, I could do that
according to more general conventions. My library already does not
strictly enforce the CF data model (it takes a more practical bent), but
this is the first time I would need to come up with storage conventions
of my own.
Thanks,
Stephan
[1] https://github.com/xray/xray
On Mon, Oct 13, 2014 at 10:52 AM, Seth McGinnis <[email protected]
<mailto:[email protected]>> wrote:
Hi Stephan,
My understanding is that the CF point of view would be that if you don't
have a data variable, you can't have auxiliary coordinates.
In my experience, if your file only contains 2D lat and lon, it's
because you want to work with them as data variables, not because
they're coordinates for something absent from the file. (That, or it's
some kind of template or fractional file that you plan to add on to
another file, but in that case I don't expect the file to be meaningful
or conformant on its own.)
Can you say a bit about why any libraries that read the file should
always consider lat & lon to be coordinates? It might work to check the
standard_name or the units, instead of looking for a coordinates
attribute.
Cheers,
--Seth
On 10/12/14 1:14 AM, Stephan Hoyer wrote:
> In the process of writing a routine to faithfully serialize data
> according to CF conventions, I have encountered a conundrum.
>
> Suppose I have a would like to save a netCDF file representing a
spatial
> grid, with 1D variables "x" and "y" and 2D variables "latitude" and
> "longitude". All of these variables should be considered
coordinates by
> any libraries that read the file, and I would like to indicate this in
> the netCDF file.
>
> If there was an additional 2D data variable (e.g., "temperature"), I
> could indicate that latitude and longitude are coordinates by adding
> them to the "coordinates" attribute on the "temperature" variable.
> However, I have no such variable here.
>
> Is there any standard way I can indicate that these variables are
> coordinates if there are no non-coordinates on which to add the
> appropriate attribute? I am considering considering using a global
> "coordinates" attribute but that is definitely outside the CF spec.
>
> Thanks,
> Stephan
>
>
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