tommy's scratchpad

Not the most exciting topic, but I’ve found myself knee deep in metadata standards as they pertain to CSW in the last couple of weeks.

I’ve made some recommendations in the past for OWS metadata, which have helped in established publishing requirements for cataloguing.

Starting to look at ISO metadata (data, service) makes you quickly realize the pros and cons which come with making a standard flexible and exhaustive.  Let’s take 19139; almost everything in the schema is optional.  I think this is where profiles (such as ISO North American Profile) start to become especially important.

19119 is in the same boat.  Aside: then you start to wonder about the overlap between 19119 and OWS Capabilities metadata.  Wouldn’t it be nice if GetCapabiilties returned a 19119 document instead?  Which could plop nicely in a CSW query response as well. Oh wait, it already does.  But then try to validate the document instance.  You’ll find that OGC CSW and ISO use different versions of GML (follow the refs in the .xsd’s you’ll see them soon enough), yet apply them to the same namespace.  So validation fails.  Harmonization required!

Having said this, this is very complicated metadata which can be addressed by intelligent tools.  Tools that:

• integrate with GIS systems which can automagically populate by:
  • fetching spatial extents
  • fetching reference system definitions
  • establish hierarchy (this would be tough as it would be tied to the data management of the system)
  • fetch contact information from a given user profile (how about getting this from the network’s email / global address book against the logged in user?)

Then again, what about keeping it simple and mainstream friendly?  The toughest part is metadata creation, so let’s make it as easy as possible to do so!

How do your activities try to make metadata easier to create?

I’ve been spending alot of time lately doing a CSW client library in python, which was committed today to OWSLib.  CSW requests can be tricky to construct correctly, so this contribution attempts to provide an easy enough entry point to querying OGC Catalogues.

At this point, you can query your favourite CSW server with:

>>> from owslib import csw
>>> c = csw.request('http://example.org/csw')
>>> c.GetCapabilities() # constructs XML request, in c.request
>>> c.fetch() # HTTP call.  Result in c.response
>>> c.GetRecords('dataset','birds',[-152,42,-52,84]) # birds datasets in Canada       
>>> c.fetch()
>>> c.GetRecords('service','frog') # look for services with frogs, anywhere
>>> c.fetch()

That’s pretty much all there is to it.  There’s also support for DescribeRecord, GetRecordById and GetDomain for the adventurous.

I hope this will be a valuable addition.  Because CSW uses Filter, I broke things out into a module per standard, so that other code can reuse, say, filter for request building and response parsing.  A colleague is using this functionality to write a QGIS CSW search plugin.

My next goal will be to put in some response handling.  This will be tricky given the various outputSchema’s a given CSW advertises.  For now, I will concentrate on the default csw:Record (a glorified Dublin Core with ows:BoundingBox).

So try it out;  comments, feedback and suggestions would be most valued.

Oh ya, thank you etree!