[OSGeo-Standards] WMS-C and Capabilities

Christopher Schmidt crschmidt at metacarta.com
Tue Aug 26 12:57:03 EDT 2008

On Tue, Aug 26, 2008 at 05:19:06PM +0200, Andrea Aime wrote:
> Christopher Schmidt ha scritto:
> >On Tue, Aug 26, 2008 at 10:36:37AM +0200, Andrea Aime wrote:
> >>Jody Garnett ha scritto:
> >>>Just a quick confirmation; the WMS-C page here:
> >>>- http://wiki.osgeo.org/wiki/WMS_Tiling_Client_Recommendation
> >>>
> >>>Has confused a couple implementations; my understanding is this 
> >>>specification talks about adding to the VendorSpecificCapabilities of an 
> >>>existing WMS server a section describing the available TileSets.
> >>>
> >>>Can we change the "Example VendorSpecificCapabilities" to make it really 
> >>>obvious what is expected...right now I have servers returning:
> >>>http://sigma.openplans.org/geowebcache/service/wms?request=getcapabilities
> >>>><WMT_MS_Capabilities version="1.1.1" updateSequence="0">
> >>>>  <VendorSpecificCapabilities>
> >>>>    <TileSet><SRS>EPSG:4326</SRS><BoundingBox srs="EPSG:4326" 
> >>>>minx="-180.0" miny="-90.0" maxx="180.0" maxy="90.0" />
> >>>>       ....
> >>>>    </TileSet>
> >>>>  </VendorSpecificCapabilities>
> >>>></WMT_MS_Capabilities>
> >>>Which is not enough information for clients (ie uDig) to fetch Titles 
> >>>and Layer information. WMS-C is not a standalone server; it is an 
> >>>additional ability added to an existing WMS server.
> >>Hum, reality seems to think otherwise, both TileCache and GWC are
> >>born as standalone servers, they cannot serve non tiled requests.
> >>GWC can be embedded in GeoServer as a convenience, but it does not
> >>act as an extension to WMS.
> >
> >Reality or not, the Capabilities were designed to be in addition to
> >standard WMS capabilties. The fact that TileCache doesn't do non-tiled
> >requests doesn't mean that the WMS information isn't neccesary: most of
> >the metadata was always supposed to be delivered by the 'traditional'
> >WMS GetCaps, and excluding it from your response is (imho) buggy.
> >
> >(I don't *think* TileCache does that, but if it does, it should be
> >fixed.)
> I did not explain myself. I do not mean to say the capabilities document
> should be partial or anything, I just mean the client cannot expect
> a tiling server to answer any kind of request like a WMS would do.
> The capabilities document is an extension all right, but the behaviour
> is a restriction, only certain GetMap requests will be answered, no?
> Put in other words, the caps document is based on WMS, but the tiling
> server is not a valid WMS server, right?

In the general case that's true, but that's not required. TileCache with
PIL could certainly answer non-tiled requests, I just don't care to make
it do so. (Similarly, TileCache has no reprojection capabilities,
because I've said that it shouldn't, I expect.)   

Christopher Schmidt

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