I have a points feature class that I am trying to get elevation values for the state of texas. ArcGIS Online conveniently has a DEM for the entire state as a WMS. I am wondering if there is a way to extract the elevation values from the WMS to a field in my points feature class. In QGIS I have tried using the point sampling tool but it does not seem to recognize the pixel values in the WMS. When I id a pixel the elevation is displayed through an html element. Is there any possible way to extract this value?
What you want is not possible. The info or id tool you use to get a single value for the point you clicked on the map is sending a GetFeatureInfo Request for this coordinate to the server and the server sends back an xml/html file(which is in general optional, as well as the "elevation-awareness" of this wms from ESRI). But only for this single point. Even if you could extract it from the displayed element you would have to click each point (or write some program that does this, which is unlikely to work as they would block you pretty fast from the server). A WMS is not a DEM or any other data, its only a picture to look at. It is desgined that way to not give out the data... if that would be the idea you would have another service for it.
I'm sorry but I'don't agree with you a wms layer is not a simple picture, is a vector without the geometry. If you click with info on a wms layer you can see the value of the field that show the layer.
Nowadays in Italy we get info from wms of cadastral but with QGIS, I'm looking for the same solution for Arcgis.
You can see there, also if it is in italian