As the World Wide Web has developed into a ubiquitous information source, links to on-line information and services have become critical elements in all metadata dialects. Some dialects emerged during the early days of the web when less was known about how it would develop and flourish. URLs were simple and self-explanatory and it was enough to include just the bare URL in the metadata. As URLs have increased in complexity, it has become more important to provide supporting information along with the links.
URL's are described in DIF using the Related_URL field. They have the following properties:
<dif:Related_URL uuid="UUID"> <dif:URL_Content_Type/> <dif:URL/> <dif:Description/> </dif:Related_URL>
The URL_Content_Type field comes from the URL Content Type Keywords which provide standard names for a number of data systems and services. Data access URLs can be recognized by the URL_Content_Type = "GET DATA". The Related_URL field is highly recommended and may be repeated.
The ECHO model includes several types of URLs, each with a unique set of properties:
<OnlineAccessURL> <URL/> <URLDescription/> <MimeType/> </OnlineAccessURL> <OnlineResource> <URL/> <Description/> <Type/> <MimeType/> </OnlineResource> <ProviderBrowseUrl> <URL/> <FileSize/> <Description/> <MimeType/> </ProviderBrowseUrl>
The ISO Standards use CI_OnlineResources to describe links. They include the following properties:
<gmd:CI_OnlineResource> <gmd:linkage/> <gmd:protocol/> <gmd:applicationProfile/> <gmd:name/> <gmd:description/> <gmd:function/> </gmd:CI_OnlineResource>
All of these approaches to describing online resources include properties that make links more self-explanatory and easier to use. There are some differences that need to be considered when comparing them: