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Peter Baumann wrote:
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.Steve- thanks a lot for taking time on this. Some questions inline: Steve Hankin wrote: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.)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".
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 "Si forte in alienas manus oberraverit hec peregrina epistola incertis ventis dimissa, sed Deo commendata, precamur ut ei reddatur cui soli destinata, nec preripiat quisquam non sibi parata." (mail disclaimer, AD 10xx)
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