[Users] DescribeLayer and DescribeFeatureType

[Bart van den Eijnden (OSGIS)]

Hi list,

today Andreas and I had an interesting discussion about WMS 
DescribeLayer and WFS DescribeFeatureType.

Let me first give a bit of background for people not familiar with these 
requests.

WMS DescribeLayer is currently part of the SLD WMS spec, and is meant to 
tie a WMS layer to a WFS typename or WCS coverage. Not all WMS services 
implement this though, Mapserver and Geoserver do, but e.g. ArcGIS 
Server currently does not. There are plans to move the DescribeLayer 
request from SLD WMS to mainstream WMS in WMS 1.4. A simple parser for 
WMS DescribeLayer response is in [1].

Currently a lot of applications by-pass WMS DescribeLayer and assume WMS 
OnlineResource = WFS OnlineResource and WMS layer name = WFS typename. 
Although this will work in some cases, it will fail in others, but it 
saves a trip to the server. So a question for GeoExt is: do we want to 
follow the OGC way in this? I would be in favour of using WMS 
DescribeLayer even if this is an extra round-trip to the server. But 
what do others think?

WFS DescribeFeatureType is a way to get an XML Schema back describing 
the structure of a typename, to put it more simple it will return a list 
of attributes from a layer. This can be used then to e.g. fill combo 
boxes for layer attributes, for a querybuilder component for instance. 
Btw a WFS DescribeFeatureType parser was just committed to OpenLayers 
trunk this weekend by Andreas [2].

The solution to integrate this into GeoExt that Andreas just came up 
with is everytime a WMS layer is added to the LayerStore, we will 
perform a DescribeLayer and subsequently a WFS DescribeFeatureType 
request and store this information in the LayerStore.

Does this approach make sense? Thoughts?

Best regards,
Bart & Andreas

[1] http://trac.openlayers.org/ticket/1201
[2] http://trac.openlayers.org/ticket/1202

-- 
Bart van den Eijnden
OSGIS, Open Source GIS
bartvde at osgis.nl
http://www.osgis.nl