[MapProxy] Troubles creating a proper configuration for german wmts service

[Malte Zacharias]

Hi folks,

I'm currently trying to setup MapProxy as a WMS cache for a wmts service. The german authorities provide a free-for-private-use service with a complete official german map.

Unfortunately this is wmts only and I also want to leverage the cache feature for possible offline usage. I'm trying to configure mapproxy to do this but I'm failing because with 
my configuration the wrong tiles are requested. In zoom level 0 for example tile 0,0 is requested, but the service returns a 404 error. I'm pretty sure that there needs to be some 
kind of offset in the tile number, but I don't know how to configure that. I've attached the WMTSCapabiliites file (I had to remove my user credentials as license agreement states
 that I may not publish those, but if you want to try it out you can register for a free user id at https://upd.geodatenzentrum.de/auftrupd/gdz_webatlas.wade?gdz_kz=0&gdz_service=&gdz_kontakt=&gdz_kontaktr=&gdz_spr=deu  german language only) 
Anyway, I hope my question can be answered based on the Capabilities file alone.

I am pretty new to all this, so I'm having trouble understanding, how to configure mapproxy properly. This is my configuration so far (only changes from the standard demo config file):

------------------------------------------------------------------------------------------
layers:
  - name: geoserver
    title: geoserver
    sources: [osm_cache]

caches:
  osm_cache:
    grids: [mygrid]
    sources: [geoserver]

sources:
  geoserver:
    type: tile
    grid: mygrid
    url: https://sg.geodatenzentrum.de/wmts_webatlasde__<user-id removed>/tile/1.0.0/Webatlas/default/default028mm/%(z)s/%(y)s/%(x)s.png
    http:
      ssl_no_cert_checks: true # going to change this later
    on_error:
      404:
        response: '#DD7777' # just to highlight the 404 errors
        cache: False
    coverage:
      bbox: [280387.70990000013, 5193471.164086349, 981683.1163957692, 6137570.076873273]
      bbox_srs: EPSG:25832

  mygrid:
    srs: EPSG:25832
    tile_size: [512, 512]
    bbox: [280387.70990000013, 5193471.164086349, 981683.1163957692, 6137570.076873273]
    origin: ul
    res: [
         #  res            level     scale @91.0 DPI
         2637.5261643119, #  0     9449404.76190476
         1978.1446232339, #  1     7087053.57142857
         1318.7630821559, #  2     4724702.38095238
          791.2578492936, #  3     2834821.42857143
          527.5052328624, #  4     1889880.95238095
          395.6289246468, #  5     1417410.71428571
          263.7526164312, #  6      944940.47619048
          197.8144623234, #  7      708705.35714286
          131.8763082156, #  8      472470.23809524
           79.1257849294, #  9      283482.14285714
           39.5628924647, # 10      141741.07142857
           26.3752616431, # 11       94494.04761905
           19.7814462323, # 12       70870.53571429
            7.9125784929, # 13       28348.21428571
            5.2750523286, # 14       18898.80952381
            3.9562892465, # 15       14174.10714286
            2.6375261643, # 16        9449.40476190
            1.9781446232, # 17        7087.05357143
            1.3187630822, # 18        4724.70238095
            0.7912578493, # 19        2834.82142857
    ]
------------------------------------------------------------------------------------------


I also know that for example https://sg.geodatenzentrum.de/wmts_webatlasde__<user-id removed>/tile/1.0.0/Webatlas/default/default028mm/7/44/54.png is a valid URL, 
and if I open the demo page viewer I can actually zoom in to level 7 and pan the map around until I finally get to the tile 44/54 and those are displayed properly along with the 
surrounding tiles. If I zoom in or out however I'm again not at the right tile coordinates. My best guess currently is, that I somehow have to factor in the TopLeftCorner from the TileMatrix part in the xml:

          <TileMatrix>
          <ows:Identifier>0</ows:Identifier> 
          <ScaleDenominator>9449404.761904761</ScaleDenominator>
          <TopLeftCorner>-5120900.0 9998100.0</TopLeftCorner>  
          <TileWidth>512</TileWidth> 
          <TileHeight>512</TileHeight>
            
            <MatrixWidth>2</MatrixWidth> 
            <MatrixHeight>2</MatrixHeight>
          
          </TileMatrix>

I'd appreciate your help on this, as I hope it's not much that is left...

Thanks
Malte
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