TMDM-NG – Overloading Occurrence

“Occurrence” in topic maps is currently overloaded. Seriously overloaded.

In one sense, “occurrence” is used as it is in a bibliographic reference. That is that subject X “occurs” at volume Y, page Z. A reader expects to find the subject in question at that location.

In the overloaded sense, “occurrence” is used to mean some additional property of a subject.

To me the semantics of “occurrence” weigh against using it for any property associated with a subject.

That has been the definition used in topic maps for a very long time but that to me simply ripens it for correction.

Occurrence should be used only for instances of a subject that are located outside of a topic map.

A property element should be allowed for any topic, name, occurrence or association. Every property should have a type attribute.

It is a property of the subject represented by the construct where it appears.

Previously authored topic maps will continue to be valid since as of yet there are no processors that could validate the use of “occurrence” either in the new or old sense of the term.

Older topic map software will not be able to process newer topic maps but unless topic maps change and evolve (even COBOL has), they will die.

3 Responses to “TMDM-NG – Overloading Occurrence”

  1. What you write is absolutely true. This error arose in the original 13250, and we kept it for 13250-2:2006. As Michael Kay says, “backwards compatibility means deliberately repeating other people’s mistakes.” Of course, when one does that, it’s for a reason.

  2. Interesting thought, – I agree in principle.

    But if you want to change the TMDM-spec, why not let occurrence be a property-type? This would simplify the model. In principle this would mean to rename “occurrence” to “property” and add special semantics to the type of occurrences that are “real occurrences” (URIs with special semantics).

    Wouldn’t this make the transition from the old to the new model easier?

    occurrence -> property

    Representation of a relationship between a subject and
    an information resource

    occurrence type -> property type


    Property type for Instances of a subject that are located outside the topic map.

    This would also however lessen the priority given to the “Occurrence”-semantics in the current TMDM. People are used to hearing about the Topic Maps TAO (Topics, Associations, Occurrences) as the core of the model.

    The TAO-term was probably a bit more effective marketing than Steve Pepper intended, as he now officially would like people to stop reading his TAO-paper and move on to some newer information about Topic Maps, ref recent posting on the topicmapmail-list:

    Another thought:
    I think this also would get rid of “internal occurrences”. – A bonus, as this term is a bit confusing to explain to people. Ref chap 3.5 in “Towards a methodology for developing Topic Maps ontologies”:

  3. Patrick Durusau says:


    That is certainly one solution.

    Personally I would rather model occurrences as associations and simply use “occurrence” as a name for a particular topic type.

    Avoiding the notion of an “internal” occurrence is certainly a bonus.

    Rather than lessening the usefulness of the TAO I would think it would increase it. Because there we would not have the odd case where occurrence really doesn’t mean occurrence (in the usual sense of the word).