|Ram Kumar||Aug 17, 2006 6:53 pm|
|Subject:||Fwd: Question about GML|
|From:||Ram Kumar (kuma...@gmail.com)|
|Date:||Aug 17, 2006 6:53:27 pm|
Message from OGC about their use of XLINK.
And here is some guidance from Ron Lake, father of GML and XML geek extraordinaire.
There are some xlink tools - but none of them are very recent nor very useful. Any XML parser will parse the xlink attributes. The question is then what semantics you apply to them.
The meaning of xlink:href on a GML property is that it is a pointer to the value of the property. It plays a similar role to the rdf:resource property in RDF. Parsing the value of the xlink:href (a URI) in XML is again handled by the parser. The value of the URI may be an arbitrary URI that is a URL and points to the resource just like in HTML. It may be a URN (Uniform Resource Name) that you can use as if it were well known (i.e. do a table look up in your own software) or it may be passed to a URN resolver that does the look up for you and provides the resource or a handle to the resource (perhaps a URL) that use to get the resource. So for simple xlinks (most of the usage in GML) you don't meet much in terms of special software.
You may also want to make use of some of the other properties in the gml Association Attribute Group - in particular gml:remoteSchema. Parsing is just as above - but you need to use the referenced resource (a schema fragment pointer) you will need to fetch the resource as above, apply the likely XPointer filter contained in the URI value, and then pass the schema fragment to an XML Schema validating parser. Again no really special tools involved.
Where life could get more complex is if the xlink:href or gml:remoteSchema attributes contain XPointer expressions - especially ones that use string operators etc. Here there are a number of libraries around that can be used as starting points. Including:
I hope that helps! BTW there are also some libraries and XLink processing engines (see http://skew.org/~mike/4suite-docs/html/modules/Ft.Xml.XLink.html)
Thanks for the pointers. Will have a look. We are having problems with XLINK as there is no support for XLINK in industry and organisations hat implement xlink, have their own implementations and this is resulting in problems with interoperability. CIQ V3.0 is struggling due to this as XBRL wants to interoperate with CIQ V3.0 and both these specs. have different implementations of XLINK. I am sure this will be the case for GML also as W3C does not specifiy a schema for XLINK.
On 8/18/06, Carl Reed OGC Account <cre...@opengeospatial.org> wrote:
Hi Ram -
All is going well, thank you.
Yes, I believe that you can use Xlink with GML. But, not being a XML or GML geek, I do not know the answer to your question. So, I will pass the question onto the GML gurus.
In terms of a simple GML profile, a couple of updates:
1. The OGC membership has approved the GML Simple Features Profile. This is a simple - but rich - profile of GML that probably provides more capability than is perhaps required to solve many of the location encoding situations in OASIS. However, check it out at http://portal.opengeospatial.org/files/?artifact_id=15201 .
2. For the majority of simple geometry encoding requirements (point, line, etc), we are working the GML OASIS Profile. I worked up a schema last week for use by Sukumar in the HAVE draft standard. However, it does not quite validate, so I have asked the GML gurus to fix the schemas for me.
3. And of course, there is the really simple GML encoding used in GeoRSS - which is the basis for the GML OASIS Profile. Check out www.georss.org.
Also, thought you might be interested to know that the latest version of Google Earth KML uses xAL for expressing Addresses: http://earth.google.com/kml/kml_tags_21.html
How are you? I have a question for you about GML. I understand GML uses XLINK. What sort of tools are used to parse XLINK related docs? Any pointers will be great.
As soon as your group has produced the XML schemas forcsimplified version of GML, please let us know.
Ram Kumar OASIS CIQ TC