[topicmapmail] Difference between SubjectIdentity & Occurence

[Steve Pepper]

Dear Prem,

At 12:47 14.04.2003 +0530, Premnath Raghavendran wrote:
>Am new to Topic Maps and having problems in understanding when exactly to 
>use SubjectIdentity and Occurence tags.
>
>Both these tags seem to be interchangeable. Can someone give me a clear 
>picture of when to use which tag.
>
>For example, in the following Topic definition what is the difference 
>between Occurence & SubjectIdentity ? How do we generally define what is 
>an Occurence & what is a SubjectIdentity?
>
>
><topic id="sw-lukeskywalker">
>  <instanceOf>
>   <topicRef xlink:href="#sw-person" />
>  </instanceOf>
>  <baseName>
>   <baseNameString>Luke Skywalker</baseNameString>
>  </baseName>
>  <subjectIdentity>
>   <subjectIndicatorRef 
> xlink:href="<http://www.starwars.com/databank/character/lukeskywalker/index.html>http://www.starwars.com/databank/character/lukeskywalker/index.html" 
> />
>  </subjectIdentity>
>  <occurrence>
>   <instanceOf><topicRef xlink:href="#picture"/></instanceOf>
>   <ResourceRef? 
> xlink:href="<http://starwars.com/photos/lukepicture.html>http://starwars.com/photos/lukepicture.html" 
> />
>  </occurrence>
></topic>

The purpose of the <subjectIdentity> element is to control merging
behaviour. If two <topic>s have the same "subject identity" they are
deemed to represent the same subject and should be merged.

The <occurrence> element merely asserts a relationship between a
subject and some information resource and has no significance in
terms of merging (unless, of course, your application chooses to
give it significance).

In other words: say you had two more topics in your topic map,
"sw-lukeskywalker2" and "sw-lukeskywalker3". Let's say that the first
one has the same <subjectIdentity> as "sw-lukeskywalker" but no
occurrences, and the second one has the same <occurrence> as
"sw-lukeskywalker" but no subject identity.

A conforming topic map processor would merge "sw-lukeskywalker" and
"sw-lukeskywalker2" to a single topic, but it would not merge
"sw-lukeskywalker3".

Hope this helps.

Steve

P.S. By the way, the syntax in your example is not quite correct.
It should be as follows:

<topic id="sw-lukeskywalker">
  <instanceOf>
   <topicRef xlink:href="#sw-person" />
  </instanceOf>
  <!-- subjectIdentity *before* baseName -->
  <subjectIdentity>
   <subjectIndicatorRef 
xlink:href="http://www.starwars.com/databank/character/lukeskywalker/index.html" 
/>
  </subjectIdentity>
  <baseName>
   <baseNameString>Luke Skywalker</baseNameString>
  </baseName>
  <occurrence>
   <instanceOf><topicRef xlink:href="#picture"/></instanceOf>
   <resourceRef xlink:href="http://starwars.com/photos/lukepicture.html" />
<!--^^^^^^^^^^-->
  </occurrence>
</topic>


--
Steve Pepper, Ontopian
http://www.ontopia.net
DUMUS DELENDA EST