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For my web map application http://ian01.geog.psu.edu/geoserver/www/pa/pa3.html I have three levels of background imagery as the user zooms in. At small scales I'm using NASA Blue Marble and a large scale I have 1ft USGS orthos which are both great. However at medium scales I'm using Landsat imagery and it looks poor (to say the least), so can anyone suggest a good source of free imagery for the US (this case is PA but I need to do other states too). Preferably WMS servers.

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One option is to try another WMS of Landsat, such as the 'visible' style provided by NASA. There are a number of pan-fused 15m versions of the data, see the Landsat 7 wikipedia external links. However, I don't think any of these are free for public consumption.

Another option is to generate your own Landsat merged images with the quality you'd like, its easy enough to do: first download the tiles you're interested in (you'll want the raw bands, not just the band combos), then use something like GRASS to perform a band merge, using for example i.landsat.rgb (doc) or i.fusion.brovey (doc).

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I had hoped to stay away from oneearth due to reliability "problems" in the past but it does look promising. – iant♦ Oct 11 '10 at 17:19
I've updated my answer to give you another option: perform the merges yourself, which will give you complete control over the output, and provide a much 'cleaner' image than the heavily compressed ones freely available via IMS. – scw Oct 15 '10 at 6:21
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It looks like you are serving up from GeoServer locally for some of your data. You can download the NAIP imagery noted above, and then downsample it if your disk space requires this. If you have the disk to spare, you can just create some overviews of the data (using gdaladdo). If you just want to use someone elses WMS they will do it for you as you request tiles for those zoom levels.

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That might work - I could just down sample my existing 1ft USGS data too. But for the whole of the US I doubt I have enough disk space. – iant♦ Oct 14 '10 at 17:23
If you don't have enough disk space, then instead of creating overviews and keeping the original, you can just downsample using gdal_translate with the outsize parameter (eg -outsize 25% 25%) to get a significantly smaller image. – jlivni Oct 22 '10 at 6:12
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Although I am not sure this would constitute "medium" resolution, here is the GetCapabilities for NAIP 1m imagery for the continental US.

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I think 1m is too small I was thinking of something in the 50m - 100m range – iant♦ Oct 11 '10 at 14:11

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