[Qgis-user] select elements from a wms layer

[Richard Duivenvoorde]

José María Michia wrote:
> Hi all
> 
> I want to export a sub region of a WMS layer, but the selection tool
> is not enabled when a layer WMS is active.
> 
> Is it not possible to select elements of a WMS layer?

Hi José,
what do you exactly mean by 'export a sub region'?

A WMS-layer is a flat image. That is whenever you zoomin or pan, a new 
png/gif/jpeg image is requested from a WMS-server 
(http://en.wikipedia.org/wiki/Web_Map_Service) and shown in your map.

But this means that the underlying data is NOT available for QGIS, while 
it off course is when you are showing shapefile or postgis data.

So there is (besides exporting it to an image) not so much to export I 
think.

It is however possible (if the WMS-layer is 'queryable'), to fire 
getFeatureInfo-requests to the WMS-server. Using the I-tool, you can 
click on the WMS-layer, an x,y is sent to the WMS-server and the server 
will then sent back the attributes of one or more features that are 
laying on that x,y. This is possible because the WMS-server DOES have 
the data with which it 'paints' the map images.

Another option is to use a WFS-layer (mind the F). A webFEATUREserver 
(http://en.wikipedia.org/wiki/Web_Feature_Service) There is a plugin 
available for those.  Sometimes a WMS is also configures as a WFS.
A WFS will return 'features' (that is the geometric objects including 
attributes) as gml. Qgis is then able to generate a map based on this 
data, and the different objects are selectable.
If you want to try out a WFS: use this url:
http://www.rijkswaterstaat.nl/services/geoservices/overzichtskaartnl?service=WFS&typename=NL-prov&version=1.0.0
you will be able to receive provinces, highways etc from the Netherlands.

Regards,
Richard Duivenvoorde