1

I used to read that WMS is a stream of images, for example : image/png. And that, if you need additional data about what is underlying, you have to send WFS requests aside.

But what about WMS streams of vectors (instead of rasters). I used to open a WMS layer with QGIS, and at that time I don't provide a WFS layer with it. QGIS is able to show me attributes content of what I am clicking on with my mouse.

What does the WMS stream carries to the client then ? Images ? It has to transfer data attributes too, isn't it ?

3

Generally speaking a WMS gives singular images (or videos) on request (GetMap), so output formats will be normally, image/png, image/jpeg, image/tiff. A caveat would be use of WMS to serve MapBox vector tiles.

A WMS is not a streaming service.

The data the WMS converts into its output can be vector or raster, or some combination of those. The attribute data of the input data are not available in the GetMap response, but may be available as part of a separate request, a GetFeatureInfo request.

As WMS requests are stateless, the GetFeatureInfo request must pass the majority of the request parameters to request the map image that was clicked on, plus the coordinates of the pixel (point) that was clicked on, plus the output format for the attribute information to be returned in.

So a WMS GetMap request generated by QGIS might look like:

http://ogc.bgs.ac.uk/cgi-bin/BGS_BGS-HPA_Radon_Potential/wms?language=eng&VERSION=1.3.0&REQUEST=GetMap&CRS=EPSG:4326&BBOX=50.179445759563,-4.41340973640857,50.9615344710795,-3.2834397199341&WIDTH=877&HEIGHT=607&LAYERS=GBR_BGS-HPA_625K_RADON_POTENTIAL&STYLES=&EXCEPTIONS=XML&FORMAT=image/png&BGCOLOR=0xFEFFFF&TRANSPARENT=TRUE&

and give a map image like:

WMS GetMap request result image

and the GetFeatureInfo request would be like:

http://ogc.bgs.ac.uk/cgi-bin/BGS_BGS-HPA_Radon_Potential/wms?language=eng&VERSION=1.3.0&REQUEST=GetFeatureInfo&CRS=EPSG:4326&BBOX=50.179445759563,-4.41340973640857,50.9615344710795,-3.2834397199341&WIDTH=877&HEIGHT=607&LAYERS=GBR_BGS-HPA_625K_RADON_POTENTIAL&STYLES=&EXCEPTIONS=XML&FORMAT=image/png&BGCOLOR=0xFEFFFF&TRANSPARENT=TRUE&QUERY_LAYERS=GBR_BGS-HPA_625K_RADON_POTENTIAL&INFO_FORMAT=text/html&I=194&J=162&

That is we have replaced REQUEST=GetMap& with REQUEST=GetFeatureInfo& and added QUERY_LAYERS=GBR_BGS-HPA_625K_RADON_POTENTIAL& the layer of the map we want to query on, INFO_FORMAT=text/html& the format we want the response in, and I=194& J=162& the pixel coordinates of the point clicked on.

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