Ontology Mapping

Greetings,

===Sorry for the repost.  I didn't get any feedback to the first posting.===

I would welcome any advice/comments on Ontology Mapping.  I am trying to put together an exemlar that demonstrates how an Articulation Ontology works.

My question is in reference to Chapter 4 in the Web Ontology Language Guide Version 1.0 dtd 10 Feb 2003 Ontology Mapping.

Assuming you have two separate ontologies (O1, O2) both having separate conceptualizations you wish to adopt.  In order to extend the desired conceptualizations,  a separate Articulation Ontology is established to articulate/map the desired conceptualizations from both sources.  In order for this concept to work would the Articulation Ontology first be required to import O1 and O2 before declaring that O1 ClassA equivalentClass  O2 ClassB ?  Or is it a better practice to precede the mapping statement in the Articulation Ontology with the URL of each equivalent class and not bother with the import allowing the URL to point to the location(s) of the equivalent classes?  

Also, according to the reference, it is more efficient to articulate as "high up" the class hierarchy as possible, meaning once it is stated that two classes are equivalent, by definition all their members must satisfactorily belong to both classes.  Would this obviate the requirement for a similar articulation at the instance level as long as the user subscribed to the higher up Articulation Ontology?  What harm would the redundancy cause if mappings at both levels occurred? 

Thanks,

I apologize for the rookie questions, but I really want to get a handle on this.
. 
Marty