Calculate the median of features (within a WMS layer) in a given map extent as dynamic text in map layout

Question

Is there a way to calculate the median of several standard land values from the following WMS service of a given map extent? I want to use it as a dynamic text via expression. I am using the Atlas Tool.

For example: 4 Zones in a given map extent (50, 60, 60 and 80 Euro per sqm) result in a median of 60 in my may Layout.

https://geodatenkatalog.frankfurt.de/geonetwork/srv/ger/catalog.search#/metadata/b3c48b5b-54d7-46b6-b703-5a2d1b4bdeec/formatters/customed-xsl-view?root=div&view=gdiffm

I have tried the following, but without success:

aggregate( 
layer:='Bodenrichtwert_Bodenrichtwertzone_34cc603d_9bb9_45d5_a63f_0f0fdf8fbaf2',
aggregate:=mean("bodenrichtwert"),
expression:=$id,
filter:=within($geometry,@atlas_geometry)
        )

Answer 1

It won't be possible, because you can't get at the full set of underlying data values though a single request via the WMS interface; you can only get data from a point location (through a GetFeatureInfo request).

For example a GeoJSON GetFeatureInfo request at pixel location i=100,j=100 in your example GetMap request gives:

  {
    "type": "FeatureCollection",
    "features": [
      {
        "type": "Feature",
        "geometry": null,
        "properties": {
          "OBJECTID": "3129",
          "gemeindename": "Frankfurt am Main",
          "bodenrichtwert": "21",
          "stichtag": "01.01.2022",
          "bodenrichtwertzoneName": "1630",
          "entwicklungszustand": "Fläche der Land- und Forstwirtschaft",
          "beitragsAbgeaberechtlicherZustand": "Null",
          "nutzungart": "Erwerbsgartenanbaufläche",
          "nutzungergaenzung": "Null",
          "gutachterausschussbezeichnung": "Gutachterausschuss für Immobilienwerte für den Bereich der Stadt Frankfurt am Main",
          "VERF": "Null",
          "WNUM": "16301252",
          "wertrelevanteGeschossflaechenzahl": "Null"
        },
        "layerName": "Bodenrichtwertzone"   
      }
    ]
  }

Sometimes there is a complementary WFS service to allow data access, but that is not available in this case