Peter Baumann wrote:
Steve-
thanks a lot for taking time on this. Some questions inline:
Steve Hankin wrote:
Hi Peter,
I'll start the ball on this with a short answer.
CF datasets are self-describing. They do not reference a controlled
vocabulary of coordinate reference systems external to the file. Thus a
CF subset of a valid CF dataset is always another valid CF dataset and
its geo-location is self-describing -- even if it has fewer dimensions
than the parent file. The question you are asking does not really apply
to CF datasets "in their native habitat".
I see. Does this inline description change during subsetting? I.e.,
when you are building 2-D slices (just as an example) is elevation and
time information removed from the embedded CRS information? (They may
well remain somewhere in a metadata description, this is not our
concern for now.)
If we imagine extracting (say) a single 2D slice in the XT (lon-time)
plane from a 4D XYZT dataset, then the Y and Z axes are conceptually
reduced to single points ("projected"). A well constructed CF dataset
will include single point axes (*) for Y and Z, just as it includes
multi-point axes for X and T. We sometimes speak of the 2D slice as
being a *degenerate* 4D dataset, since it still possesses the full 4D
coordinate machinery. The way in which the slice is embedded into the
larger coordinate system remains self-describing.
In a similar manner, an in situ observation of a vertical profile or of
a time series may simply be regarded as a degenerate 4D dataset. With
this outlook broad classes of data may legitimately be thought of as
"gridded" (though using that term would lead to confusion in some
circles). This is part of the power of the netCDF data model -- that it
unifies the representation and semantics of so many different types of
features.
(*) footnote to say that there is a also a short-hand way to represent
scalar axes in CF ... but using it doesn't alter the self-describing
nature of the file.
- Steve
Echoing your question back: CF 1.2 (??) added a section on "5.6. Grid
Mappings and Projections Horizontal Coordinate Reference Systems", which
is specifically to handle the associations between CF datasets and the
corresponding crs.
(http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.4/cf-conventions.html#grid-mappings-and-projections).
Should this mapping be refined to address solutions to 4D (n-D)
subsetting that you have developed in WCS?
Interesting to read this, it helps us to understand your approach.
Actually, we are not yet in a position to answer this question,
currently we are trying to disentangle issues of slicing functionality
and CRS handling (the former we would like to have in the core, the
latter in an extension). We will gladly come back for this reverse
discussion once we have something in our hands.
-Peter
- Steve
=============================
Peter Baumann wrote:
Hi community,
in the WCS group we are wondering how you deal with subsetting
operations in n-D data spaces - obviously a result with less
dimensions than the original cube needs to get a different CRS
associated. How do you find the appropriate result CRS, for example,
for a x/t cut from an x/y/z/t cube?
thanks in advance for any bits of wisdom,
Peter
--
Dr. Peter Baumann
- Professor of Computer Science, Jacobs University Bremen
www.faculty.jacobs-university.de/pbaumann
mail: p.baumann@xxxxxxxxxxxxxxxxxxxx
tel: +49-421-200-3178, fax: +49-421-200-493178
- Executive Director, rasdaman GmbH Bremen (HRB 147737)
www.rasdaman.com, mail: baumann@xxxxxxxxxxxx
tel: 0800-rasdaman, fax: 0800-rasdafax, mobile: +49-173-5837882
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