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Re: [dxwg] Introduce mappingFrom and mappingTo properties (#810)

From: aisaac via GitHub <sysbot+gh@w3.org>
Date: Thu, 19 Sep 2019 20:04:58 +0000
To: public-dxwg-wg@w3.org
Message-ID: <issue_comment.created-533288328-1568923497-sysbot+gh@w3.org>
@kcoyle I believe we've all agreed that it's acceptable that some characteristics of profiles will be only meant for humans, so I guess the case you describe ("it requires a human to actually read the document") would still fall in scope. 
Anyway my point is that basically the interpretation of an alignment as a directional mapping is often very difficult, and as a matter of fact, rather a human feat than a machine one. Especially in the RDF context.
Consider the set of statements:
```
ex1:c1 rdfs:subClassOf ex2:c4
ex1:c2 owl:equivalentClass ex2:c5
ex2:c6 rdfs:subClassOf ex1:c3
```
(for a real world example see https://dcmi.github.io/schema.org/mappings.html . I don't remember where it comes from, but it looks rather serious)

Is this a mapping from `ex1` to `ex2`, or from `ex2` to `ex1`? It's in the eye of the beholder, really. 
Well actually, and to be fair to @rob-metalinkage 's point, we can express both directions with two pairs of `to` and `from` statements. That would probably be correct. And to be fair to @kcoyle 's point, it is true that most mappings come to existence because someone has a specific conversion case in mind, and that has a from and to. But the end the result (I mean, the set of statements in RDF) can be used for a conversion in either direction.
So the question is, if most mappings are this way, can `from` and `to` be given meaningful semantics? I'm curious to see @rob-metalinkage 's axioms...

Perhaps a way out of it is to make it more precise, what we mean by "mapping". If we meant a conceptual crosswalk, or a set of correspondences between (meta)data elements, then it's hard to give it a direction. If it's a "specification of a conversion" (by means of a formalism whose realizations are essentially directional, like XSLT) then it probably has a direction. Maybe there's actually room for both notions.

A bit of googling gives two interesting examples, both looking quite official:
- http://www.loc.gov/marc/dccross.html - this one has a from and a to. But it's clearly marked for data conversion purposes (" The crosswalk may be used for conversion of Dublin Core metadata into MARC, for instance as a tool for developing XSLT transformations.")
- https://www.getty.edu/research/publications/electronic_publications/intrometadata/crosswalks.html - this one looks much less directional (which by the way makes it much easier to address several vocabularies at once, as it does). It's interesting to read "This crosswalk is for planning purposes only; it is an intellectual mapping. To create a technical mapping, programmers and developers should use this crosswalk for general guidance"

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