Best practices using URIs

Hi team,

As you probably know one potential confusion when using URIs to represent
objects is that convention to distinguish between the following situations
are not clearly defined:

- URIs that correspond to retrievable resources,  
- URIs that are surrogates for real world objects, 
- URIs used to identify digital objects and 
- URIs used to identify metadata objects.

It seems sensible to use the URL as a URI for retrievable resources and
we've already had a bit of discussion about using MD5 hashes for identifying
digital objects. However I am not clear on the best approach to use for
surrogates or metadata objects. For example when we transform the Artstor
data to RDF/XML from XML, we use URIs that look like URLs for both these
situations e.g.

First identifying a metadata object:
<http://web.mit.edu/simile/metadata/artstor/mediafile#3-41822000125995.jpg>
      a       art:MediaFile ;

Second a surrogate for a real world object:
<http://web.mit.edu/simile/metadata/artstor/creator#Henderson,_T._Hunter>
      a       vra:Entity , person:Person ;

Does anybody have any suggestions about a better way to do this? 

Dr Mark H. Butler
Research Scientist                HP Labs Bristol
mark-h_butler@hp.com
Internet: http://www-uk.hpl.hp.com/people/marbut/