Resource Locator - proper URL form

[PeterParslow]

The Schematron rules, Gemini2-mi19, quite reasonably enforce that the Resource locator gmd:linkage/gmd:URL element contains a properly formed URL (they only allow http, https, and ftp - but in context allowing other IETF URL schemes is probably unwise!).

This IETF requirement is implicitly imported to ISO 19115 ("Domain" in table B.3.2.5, 'linkage' row); ISO 19139 encodes URL with xs:anyURI, which allows the use of URI fragments - so this ISO 19115 constraint can't be captured by the XSD check.

So far, so good.

We even hint at it in the "Domain" of Resource locator, except by stating "Valid URL" we leave the door open to URL fragments (relative URLs, without the scheme at the start).

I think it would be even better if we mention this in the GEMINI text, for example in the "Guidance" and / or Encoding guidance.