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Why raster saved/downloaded via WMS is different from what I see on the screen?

falqnfalqn Global Mapper UserPosts: 123Trusted User
edited September 2014 in Downloading Imagery
Hi,
I'm trying to download from Spanisch geoportal WMS. I set the viewing scale/pixel size so that I could see what I want (ex. I need good quality/resolution of scale 1:200 000). Then I'm exporting raster, I set the Sample Spacing/Scale using Use Current Pixel Size option, as Export Bounds I choose All Data Visible On Screen. The saved raster is different from I saw on the screen, it's made from more detailed scale (ex 1:50 000) but in very low resolution.
I can't save good quality/resolution raster which I see on the screen...
GM request when I'm viewing on the screen:
http://www.ign.es/wms-inspire/mapa-raster?VERSION=1.3.0&SERVICE=WMS&REQUEST=GetMap&LAYERS=mtn_rasterizado&FORMAT=image/png&STYLES=default&CRS=CRS:84&BBOX=1.8217995861,41.5960671503,2.2084300489,41.79895016&WIDTH=1677&HEIGHT=880&TRANSPARENT=TRUE

GM request when it starts to download/save raster:
http://www.ign.es/wms-inspire/mapa-raster?VERSION=1.3.0&SERVICE=WMS&REQUEST=GetMap&LAYERS=mtn_rasterizado&FORMAT=image/png&STYLES=default&CRS=CRS:84&BBOX=2,41.75,2.09375,41.84375&WIDTH=512&HEIGHT=512&TRANSPARENT=TRUE

I see that the BBOX parameter is different, in second case GM is try to download raster as couple of tiles (round BBOX values) in the first case GM is downloading one big raster in scale which suits me.

Please help.
Best regards
Wojtek from Poland

Comments

  • Thea_bmgThea_bmg Global Mapper User Posts: 38
    edited September 2014
    Hi Falqn,

    Without knowing exactly how the server is processing your request when you are exporting the data, I don't know precisely why you are getting the largest-scale images as your export. One way to get the exact images you are seeing on your screen would be to use the 'export screen contents' option in Global Mapper. From the file menu, click on 'Capture Screen Contents to Image,' and select the image type you woudl like to export. Make sure that the option to 'Generate Projection (PRJ) File' is checked, as that will preserve the georeferencing for that image.

    Will this allow you to export the images you need?

    Regards,
    Thea Youngs
  • MykleMykle Global Mapper User Posts: 452Trusted User
    edited September 2014
    Thea,

    The server for the Online Data Source / Topo Maps / USA Topo Maps has similar behavior.

    Assuming you are loading maps to cover the current screen, the map that is provided depends upon the area displayed. You can wind up with 1:24,000 or 1:100,000 or 1:250,000 source map content.

    If I have 1:100,000 map content downloaded in my current screen and want to save it to a raster file, by default the dialog will be set to the highest resolution available from that server. That is normally 1m/pixel. I will wind up with 1:24,000 content that is not useable for my purpose (way too much detail, let alone a lengthy download). I have to set 16m/pixel so the results include the 1:100,000 content.

    USGS Scanned maps at 1:100,000 scale are at 10m/pixel (looking at a tfw file), so it would be nice to be able to export this scale of source map at a resolution similar to a scanned map. But if a value less than 16m/pixel is specified, you wind up with 1:24,000 source map and it's pretty crude at that resolution.

    So I have to acquire maps elsewhere to get what I'd like. I don't think that Falqn has many other options.

    I suspect that Global Mapper can get only what the map server offers, that we don't have enough fine-grained control to get what we really want.

    These tests were run under v15 and prior. I have not looked at v16 behavior yet, and I don't expect to find any changes.

    Thanks,
    Mykle
  • falqnfalqn Global Mapper User Posts: 123Trusted User
    edited September 2014
    Hey guys,
    I agree with Mykle. I think this is a general problem with WMS services... Capture Screen Content to Image is not a solution because when you need hudge area of maps.
    I have figured out that this issue is connected with current projection - in different projections Export Raster Option (with always the same resolution) gives different results.
    Because of that I prefer TMS services - with max zoom level option I can determine which level/scale I want to download.
    Do you know is there a way to determine something like "max zoom level" in WMTS services?

    Best regards
    Wojtek from Poland
  • Jasmine_BMGJasmine_BMG BMG Staff Posts: 174
    edited September 2014
    Hello Falqn,

    For a WMTS, the server will indicate what the maximum zoom level is and the export should default to that resolution. For a WMS (no tiling) sometimes the server will indicate what the resolution is - but often it will not. In this case, Global Mapper just defaults the export to 1m per pixel.

    Regards,

    Jasmine

    Edit: For spelling!
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