Metadata about Metadata for a Service That Provides Metadata
... kind of sounds like overkill, but there's reason behind the confusion!
Imagine that you're trying to find some data. In the USGIN scheme, in order to find data you search through CSW services, which are catalogues full of ISO1939 metadata documents. Each of these documents points you to a dataset or service that is somewhere, hopefully online, for you to take a look at. These CSW services are like the card catalogues of the digital era.
But we've overlooked something... how do you know where to find an applicable CSW Service? Well, you search through a CSW! Circular, yes, but the idea is that we can build a CSW Service that points you to other CSW Services, that points you to the data you want. This way, you need to know where only one CSW service lives.
Imagine 50-some State Geological Surveys, each one with it's own library. Each of those libraries has its own catalogue describing the contents of its own library. Now, imagine that you have no idea where these catalogues are... perhaps the State Surveys live deep in a dark basement somewhere hidden away in the recesses of some State Government complex. However, you do know that there is a catalog at the AASG building down the street that lists the locations of each of the individual Survey's catalogues. You then just need to know where that one AASG catalogue is in order to find all the others, and in turn find the book you want to look at.
We intend to do the same for CSW services. We're working to build a hub CSW service that you can use to search through all the CSW services we know about. With this in mind we've built an XSLT transformation that takes the GetCapabilities response from a CSW Service, and transforms it into an ISO19139 metadata record, ready to be entered into a hub CSW. With this tool, we can start generating a very effective list of all the CSW services we know about.