World imagery scale

Ice Age MarkIce Age Mark Global Mapper UserPosts: 319Trusted User
edited July 2011 in Downloading Imagery
Hello Mike,

I'm trying to get a good color aerial image of the Avebury Henge in Great Britain. I go to download online data, then to "World Imagery" (from ESRI?). I zoom to a 1:3800 scale because below that the tiles all say "map not available yet". This is "okey dokey" enough, so I download the image as a Geotiff raster image. When I then view the saved Geotiff image in GM, it still consists of gray tiles that say "map not available yet". I tried a few more times with smaller scales, but the result is the same - no image, only gray tiles. Is there a limit on the resolution that is free, like Google Earth? Do you have any idea what the problem might be?

Also, is AsterGDEM 1.5 arc second the best available DEM resolution for this location. I read a paper that studied this spot and nearby Silbury Hill with a viewshed program like GM's; and they used 5m DEM data. Where could they have gotten this?

Thanks again,

Mark

GM 12.02 (071711)

Comments

  • redbeardredbeard Global Mapper User Posts: 279Trusted User
    edited July 2011
    For downloading imagery - you might try to find out the pixel resolution of the best avail scale, and set GM to export as that. If that doesnt make sense, just respond and I'll try again.
  • Ice Age MarkIce Age Mark Global Mapper User Posts: 319Trusted User
    edited July 2011
    Hey Redbeard,

    Thanks very much for the help. The metadata for that scale was .597m, and that was what showed up as the default for export, but it didn't work. I tried 1m and it did work, so I kept backing down and .8m was the lowest that worked even though the metadata value was .597m. That's plenty good for me, as it's better than I would have gotten from NAIP here (1m).

    I don't understand, but it worked and I got what I wanted. What resolution was it really at 1:3800? Where did the .597 come from if it's .8 ? Anyway, I learned another troubleshooting trick.

    Thanks again, and best of everything good to you.

    Sincerely,

    Mark
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited July 2011
    Mark,

    The .597 is the resolution of the most detailed layer from that server anywhere in the world. However depending on where you are the image returned at that resolution could be the 'map not available' tile and not actual imagery. Global Mapper has no way of telling as either way you get a valid image.

    For better than ASTER GDEM you will have to get premium terrain data from some place like Intermap (www.intermap.com). They should have 5m terrain for England.

    Thanks,

    Mike
  • Ice Age MarkIce Age Mark Global Mapper User Posts: 319Trusted User
    edited July 2011
    Hello Mike,

    Now I understand, thanks for the explanation. As I've said before, GM is so perfect for this kind of stuff. I found a very good map of this henge on the Avebury website. I used Google Earth to zoom, and then used the actual standing stones as Lat/Long control points to locate the map. I made white transparent on the map, and viola, the map and aerial photo were one. Very cool.

    Thanks for the Intermap link as well, I'll check it out.

    Mark
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