tjn98 wrote:
Dear All,
I think there may be two distinct cases here:
1) Local cross-referencing, where it is only necessary to establish
a relationship within a well-defined grouping of files,
2) Referencing to a universal resource, such as a specific file
held on a server.
For the former, it should only be necessary that every NetCDF file
within the grouping holds the same unique identifier (this could
be the product or group name, or an ID string from a managed soure).
Satellite swath products, where they have this sort of structure,
almost always fall in the first category. In general, a user would
want to use his or her local copy of a file, rather than re-download
a remote file.
This may be redundant by now, but my thoughts were that:
1) We only consider whether we can extend cross-referencing within
a local scope,
2) All related files within the scope should contain the same unique
identifier, perhaps a global attribute named something like
“cross_reference_ID”.
3) Referenced variable names within the scope should be unique and so
do not need modifiers. An alternative is that modifiers are not
needed in references by default, but could be included to
disambiguate variables - perhaps in a form like “geo:latitude”
where geo.nc is the file containing the required latitude variable.
If the attribute contains an empty string or is absent, CF compliant
systems only look for referenced variables within the same file, as
at the moment. If present, the system is allowed to search other files
within a limited scope, containing the same ID.
One possibility is that scope could be modified with, perhaps,
unix-like relative directory prefixes to the ID, so that
:cross_reference_ID = “my_unique_id”;
refers just to to files in the same directory, whereas
:cross_reference_ID = “../*/my_unique_id”;
refers to all files held under the parent directory and its
subdirectories, and so on.
If the purpose of the ID is only to disambiguate local files, then
form and integrity of the ID string itself could probably be left
to the discretion of the data provider, since it would only need to
be checked within a defined scope. More rigorous implementations
are a bit beyond my experience.
Anybody who’s interested can find the SAFE format definition at
earth.esa.int/SAFE. You should probably enjoy UML diagrams to
appreciate it fully. Note that the format doesn’t discuss NetCDF
in particular – this is just the format that Sentinel-3 is adopting
for its data containers.
Tim.
Hi Tim:
The only thing i saw in SAFE was referencing an absolute path HTTP URL. Do you
know if there's anything more along the lines you are suggesting?
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