On 8 March 2018 at 18:02, Richard Light <rich...@light.demon.co.uk> wrote:
> I was thinking last night that maybe we should focus our RDF efforts on > exactly this issue: the representation of the CRM primitive classes E60, > E61 and E62 in RDF. The current RDF document is becoming quite > wide-ranging in its scope, and (for example) you have questioned whether > certain sections belong in it. If we concentrate on this single aspect of > the broader RDF issue, I think we can produce something which is of > practical value relatively quickly. In particular, I would like to devote > time to this during the Lyon meeting. > I applaud the idea of focusing narrowly on something so as to produce some of practical value quickly! But I do hope that the other issues raised in that document will not be set aside too long, or lost. In particular, it seems to me that the mapping from the CRM's "properties of properties" to RDF is actually a more serious gap. In the CRM, there are a number of properties which are themselves the domain of properties. In RDF, however, a property does not have properties of its own. Incidentally, I remember years ago being able to model this directly in ISO Topic Maps, but practical considerations of interoperability and community dictate that RDF, despite its simpler model, is the technology of choice today. One example of the issue is how to model the role that individuals play in events. If a concert performance X was P14 carried out by person Y, then this maps naturally to an RDF triple in which the predicate is crm:P14_carried_out_by. However, if the person carried out that activity in a particular role (e.g. as a saxophonist) then things are more difficult. In the CRM, the P14_carried_out_by itself has the property P14.1_in_the_role_of, whose value could be an instance of E55_Type: Saxophonist. This is pleasingly consistent with how the CRM handles taxonomies in other parts of the model, but it is not workable in RDF because the P14_carried_out_by property cannot itself have a property. There are a number of "work-arounds" to this issue, such as simplying ignoring the problem and "dumbing down" the data, or moving the locus of classification from the property to the property value (e.g. in this case that would mean classifying the person rather than their role; that doesn't work very well because people may have many distinct roles, but it works better for other cases). The existing guidance would suggest defining a new "saxophone-played-by" property to be a rdfs:subpropertyof P14_carried_out_by. This can certainly work, but it's actually a poor expression of the CRM's model. It negates the practical benefits of having external taxonomies for this kind of classification. This guidance, in my opinion, makes too much of the apparent similarity between the CRM's properties and RDF properties. They are not in fact the same kind of thing, and a property which itself bears properties is more closely approximated in RDF not as a property but reified as a subject resource in its own right. A more faithful mapping of the CRM's abstract model to RDF would introduce a new RDFS class corresponding to the performance of the activity. We could then say that concert performance X was P14a_performed_in Performance Z; that Performance Z was P14b_carried_out_by person Y, and that Performance Z was P14.1_in_the_role_of Saxophonist. That's just one example of the general problem; there are a number of others, which are listed here in the context of the Linked Art project: https://github.com/linked-art/linked.art/issues/55 along with a variety of options for dealing with the issue. In my opinion the current situation with respect to properties of properties (in RDF) is really quite unsatisfactory and could be substantially improved by a more consistent treatment across the entire schema. -- Conal Tuohy http://conaltuohy.com/ @conal_tuohy +61-466-324297
