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Often WMS servers will not give you anything but a transparent image at lower zooms. When using WMS servers on the web this can often lead to a poor user experience.

I would like to build a local cache of the missing lower level zooms of the remote WMS server, so that we can also display map tiles on the web when zoomed out. I do not control the remote server so we can assume it doesn't have any special features.

What is an easy way to pull down the lowest resolution available from the WMS and then building a pyramid of the missing lower zoom level raster tiles as static files (preferably .mbtiles)?

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    depends on the terms of service - if its a GeoServer you could just ask for the .mbtiles directly. In general you are describing a WMTS service – Ian Turton Jun 25 at 19:17
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    A WMS has no concept of a zoom level, you control your own zoom levels by making requests where you specify the size of the map (in pixels) and the size of the bounding box. You won't be able to request mbtiles if the remote server doesn't offer / advertise them in the GetCapabilities response. You might want to look at solutions where you cascade the remote service through your own service (for example GeoServer and MapServer can do this), or otherwise proxy the remote service (MapProxy). – nmtoken Jul 3 at 8:30
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One way of doing it (using MapProxy, as recommended by nmtoken, but not necessarily for proxying, just for downloading):

  • Open the WMS capabilities XML and look at the MinScaleDenominator value for the layer.
  • Convert the MinScaleDenominator value to zoom level using the table at https://wiki.openstreetmap.org/wiki/MinScaleDenominator
  • Get Mapproxy. I used the Docker image. Execute the mapproxy-seed command to seed tiles into a .mbtiles files specifying a single zoom level.
  • Use gdaladdo to build the missing overview zoom levels, then serve the tiles using any mbtiles capable web server.

Sample MapProxy configuration snippet:

layer:
  - name: mylayer
    sources: [mylayer_cache]
sources:
  mylayer: 
    type: wms
    req: 
    url: <wms base url>
    layers: <wms layer name>
caches:
  mylayer_cache:
    sources: [mylayer]
    grids: [webmercator]
    cache:
    type: mbtiles
    filename: mylayer.mbtiles
seeds:
  mylayer:
    caches: [mylayer_cache]
    levels: [5] # zoom level converted from MinScaleDenominator
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