I have an XML schema file which references a urn based location. Is there some way to resolve this into a url, or some way to actually read the file it refers to.
<xs:include schemaLocation="urn:oasis:names:tc:dita:xsd:tblDeclGrp.xsd:1.1"/>
Answer
This question is not a good place for explanations about "URN resolution"... We can imagine a translation (by an automatic translator) from this oasis URN to a schemaLocation filename or a URL, but it is a little bit exotic application for URNs.
See the usual semantic and values for schemaLocation at http://www.w3.org/TR/xmlschema-1/#schema-repr
#URN resolution
You can see real-life examples of URNs at Wikipedia and the complete semantic of the term "resolve a URN" at RFC-2169 and RFC-2483. According these RFCs what you need is a local software or a URL of a "URN resolver" software (a webservice) that do N2L: given a URN, return a URL.
Wide use URNs
In 2002 the URN resolution mechanism received more attention, and two new RTFs, RFC-3404 and RFC-3405, solved a latent problem of URNs,
If URN resolution is collapsed into generic URI resolution, URNs may suffer by the lack of adoption of URI resolution.
The solution is to allow for shortcutting for URN resolution. (...) generic URI resolution starts by inserting rules for known URI schemes into the 'uri.arpa.' registry. For the 'URN:' URI scheme, one of the rules found in 'uri.arpa.' would be for the 'urn' URI scheme. This rule would simply delegate to the 'urn.arpa.' zone for additional NAPTRs based on the URN namespace. Essentially, the URI Resolution Rewrite Rule for 'URN:' is the URN Resolution Application's First Well Known Rule.
Some resolution problems persist: URNs like urn:doi or urn:lex:br have a so used and stable resolution mechanism (at dx.doi.org and lexml.gov.br/urn respectively), but is not registered at IANA's assignments/urn-namespaces; a not used, not stable and not controlled URN like urn:oasis:names:tc:dita have urn:oasis registered at IANA.