[MapProxy] Multiple Sources Presented as Single Layer

[Alberto Ponte]

We have two sources which we would like to present as a single layer.
The first source should be responsible for zooms 0-15 while the last
should be zooms 16-18.

Based on the current configuration, zooms 0-14 and 16-18 work as
expected but zoom 15 requests tiles from both sources.  I'm guessing
my min_res/max_res values in cache are off.  Can anyone shed some
light?

Also, if I wanted to seed this, would I just seed the sat_cache_out cache?

Here are the resolutions I used for EPSG:3857 zooms 0 - 18:

       43.8320494998 #  0      156543.03392800
       21.9160247499 #  1       78271.51696400
       10.9580123750 #  2       39135.75848200
        5.4790061875 #  3       19567.87924100
        2.7395030937 #  4        9783.93962050
        1.3697515469 #  5        4891.96981025
        0.6848757734 #  6        2445.98490513
        0.3424378867 #  7        1222.99245256
        0.1712189434 #  8         611.49622628
        0.0856094717 #  9         305.74811314
        0.0428047358 # 10         152.87405657
        0.0214023679 # 11          76.43702829
        0.0107011840 # 12          38.21851414
        0.0053505920 # 13          19.10925707
        0.0026752960 # 14           9.55462854
        0.0013376480 # 15           4.77731427
        0.0006688240 # 16           2.38865713
        0.0003344120 # 17           1.19432857
        0.0001672060 # 18           0.59716428

Here is the configuration:

layers:
  - name: Satellite
    title: MapLarge Satellite
    sources: [sat_cache_out]

caches:
  sat_cache_in_low:
    grids: [low_zooms]
    sources: [sat_low_zooms]
    meta_size: [1,1]
    meta_buffer: 0
    cache:
      type: sqlite
      dirname: /var/lib/mod_tile/mapproxy_satellite/low_zooms/
  sat_cache_in_med:
    grids: [med_zooms]
    sources: [sat_med_zooms]
    meta_size: [1,1]
    meta_buffer: 0
    cache:
      type: sqlite
      dirname: /var/lib/mod_tile/mapproxy_satellite/med_zooms/
  sat_cache_out:
    grids: [GLOBAL_GEODETIC,GLOBAL_WEBMERCATOR]
    meta_size: [1,1]
    meta_buffer: 0
    sources: [sat_cache_in_low,sat_cache_in_med]
    cache:
      type: sqlite
      dirname: /var/lib/mod_tile/mapproxy_satellite/out/

sources:
  sat_low_zooms:
    # zooms 0 - 15 FROM NATIONALMAP.GOV (TNM
https://viewer.nationalmap.gov/help/3.0%20TNM%20Base%20Maps.htm)
    type: tile
    url: http://basemap.nationalmap.gov/arcgis/rest/services/USGSImageryOnly/MapServer/tile/%(z)s/%(y)s/%(x)s
    grid: low_zooms
  sat_med_zooms:
    # zooms 16-18 @ 100% from NAIP
    type: wms
    wms_opts:
      version: 1.3.0
    req:
      url: http://ec2-54-161-169-51.compute-1.amazonaws.com/wms?map=/data/map/mapfiles/naip_docker.map&service=WMS
      layers: utm10-100pct

grids:
  low_zooms:
    srs: EPSG:3857
    max_res: 2.38865713
    min_res: 156543.03392800
    origin: ul
  med_zooms:
    srs: EPSG:3857
    max_res: 0.29858214
    min_res: 2.38865713